diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5510435d..72241f92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,18 @@ All notable changes to the Reactodia will be documented in this document.
 The format is based on [Keep a Changelog](http://keepachangelog.com/) and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+#### 🐛 Fixed
+- Fix `SparqlDataProvider.connectedLinkStats()` error on a response with unbound counts (treat a missing count as 0).
+- Fix query syntax error in `SparqlDataProvider.lookup()` when a text search is combined with a type filter due to a missing ending `.` in the built-in `SparqlDataProviderSettings.filterTypePattern`.
+- Fix text search in `OwlRdfsSettings`/`OwlStatsSettings` being rejected by Virtuoso ("SQ200: index of column in order by out of range") due to `?score` being bound through an expression instead of a literal constant, which Virtuoso does not accept in `ORDER BY`.
+
+#### ⏱ Performance
+- Optimize `SparqlDataProvider` queries to avoid catastrophic query plans on some endpoints (e.g. Virtuoso):
+  * Use `FILTER(!isBlank(...))` instead of `FILTER(isIri(...))` for the lookup;
+  * Change link type statistics query for `OwlRdfsSettings`/`OwlStatsSettings` to count incoming and outgoing links via `UNION` with an outer `sum()` instead of joining two aggregate sub-queries to avoid being rejected by the Virtuoso cost estimator ("The estimated execution time ... exceeds the limit").
+
+#### 💅 Polish
+- Display a query error details (message) when possible below error status bar in the `InstancesSearch`.
 
 ## [0.35.2] - 2026-08-08
 #### 🐛 Fixed
diff --git a/src/data/sparql/responseHandler.ts b/src/data/sparql/responseHandler.ts
index 6fb7fcc2..477ba3cc 100644
--- a/src/data/sparql/responseHandler.ts
+++ b/src/data/sparql/responseHandler.ts
@@ -616,7 +616,12 @@ export function appendProperty(
     values.push(propValue);
 }
 
-function parseCount(countLiteral: Rdf.Literal): number {
+function parseCount(countLiteral: Rdf.Literal | undefined): number {
+    // Count binding may be missing e.g. when aggregating over an empty
+    // solution group (some endpoints return an unbound value instead of 0)
+    if (!countLiteral) {
+        return 0;
+    }
     const numericCount = +countLiteral.value;
     return Number.isFinite(numericCount) ? numericCount : 0;
 }
diff --git a/src/data/sparql/sparqlDataProvider.ts b/src/data/sparql/sparqlDataProvider.ts
index fe4e258a..51e4fb93 100644
--- a/src/data/sparql/sparqlDataProvider.ts
+++ b/src/data/sparql/sparqlDataProvider.ts
@@ -665,9 +665,11 @@ export class SparqlDataProvider implements DataProvider {
         const navigateElementFilterOut = this.acceptBlankNodes
             ? 'FILTER (IsIri(?outObject) || IsBlank(?outObject))'
             : 'FILTER IsIri(?outObject)';
+        // Specifically use isBlank() as opposed to isIri() for filtering to avoid 
+        // a full scan on some endpoints (e.g. Virtuoso)
         const navigateElementFilterIn = this.acceptBlankNodes
-            ? 'FILTER (IsIri(?inObject) || IsBlank(?inObject))'
-            : 'FILTER IsIri(?inObject)';
+            ? ''
+            : 'FILTER(!isBlank(?inObject))';
 
         const foundLinkStats: DataProviderLinkCount[] = [];
         await Promise.all(connectedLinkTypes.map(async ({linkType, hasInLink, hasOutLink}) => {
@@ -887,16 +889,20 @@ export class SparqlDataProvider implements DataProvider {
 
             const linkPattern = refLinkType || '?link';
             const bindType = refLinkType ? `BIND(${refLinkType} as ?link)` : '';
-            // FILTER(IsIri()) is used to prevent blank nodes appearing in results
-            const blankFilter = this.acceptBlankNodes
+            // Specifically use isBlank() as opposed to isIri() when possible for filtering
+            // to avoid a full scan on some endpoints (e.g. Virtuoso)
+            const outFilter = this.acceptBlankNodes
                 ? 'FILTER(isIri(?inst) || isBlank(?inst))'
                 : 'FILTER(isIri(?inst))';
+            const inFilter = this.acceptBlankNodes
+                ? ''
+                : 'FILTER(!isBlank(?inst))';
 
             if (!direction || direction === 'out') {
-                unionParts.push(`{ ${refElementIRI} ${linkPattern} ?inst BIND("out" as ?direction) ${bindType} ${blankFilter} }`);
+                unionParts.push(`{ ${refElementIRI} ${linkPattern} ?inst BIND("out" as ?direction) ${bindType} ${outFilter} }`);
             }
             if (!direction || direction === 'in') {
-                unionParts.push(`{ ?inst ${linkPattern} ${refElementIRI} BIND("in" as ?direction) ${bindType} ${blankFilter} }`);
+                unionParts.push(`{ ?inst ${linkPattern} ${refElementIRI} BIND("in" as ?direction) ${bindType} ${inFilter} }`);
             }
         }
 
@@ -1161,9 +1167,7 @@ async function executeSparqlQuery<Binding>(
         const sparqlResponse = await response.json() as SparqlResponse<Binding>;
         return mapSparqlResponseIntoRdfJs(sparqlResponse, factory);
     } else {
-        const error = new Error(response.statusText);
-        (error as { response?: Response }).response = response;
-        throw error;
+        throw await SparqlResponseError.fromResponse(response);
     }
 }
 
@@ -1202,9 +1206,47 @@ async function executeSparqlConstruct(
         const parser = new N3.Parser();
         return parser.parse(turtleText);
     } else {
-        const error = new Error(response.statusText);
-        (error as { response?: Response }).response = response;
-        throw error;
+        throw await SparqlResponseError.fromResponse(response);
+    }
+}
+
+/**
+ * SPARQL response error originated from an endpoint.
+ */
+export class SparqlResponseError extends Error {
+    /**
+     * SPARQL endpoint response with an error result.
+     */
+    readonly response: Response;
+
+    constructor(
+        message: string,
+        response: Response,
+        options?: ErrorOptions
+    ) {
+        super(message, options);
+        this.response = response;
+    }
+
+    /**
+     * Makes an error for a non-OK response with the HTTP status and the beginning
+     * of the response body in the message, as an endpoint usually explains
+     * a rejected query there (e.g. a syntax or a query cost estimation error).
+     */
+    static async fromResponse(response: Response): Promise<SparqlResponseError> {
+        let details = '';
+        try {
+            const body = (await response.text()).trim();
+            const maxLength = 500;
+            details = body.length > maxLength ? body.substring(0, maxLength) + '…' : body;
+        } catch (e) {
+            /* ignore */
+        }
+        const status = `HTTP ${response.status} ${response.statusText}`.trim();
+        return new SparqlResponseError(
+            details ? `${status}: ${details}` : status,
+            response
+        );
     }
 }
 
diff --git a/src/data/sparql/sparqlDataProviderSettings.ts b/src/data/sparql/sparqlDataProviderSettings.ts
index 3b03e935..c4be0a66 100644
--- a/src/data/sparql/sparqlDataProviderSettings.ts
+++ b/src/data/sparql/sparqlDataProviderSettings.ts
@@ -616,7 +616,7 @@ const WikidataSettingsOverride: Partial<SparqlDataProviderSettings> = {
         }
     `,
     filterRefElementLinkPattern: '?claim <http://wikiba.se/ontology#directClaim> ?link .',
-    filterTypePattern: '?inst wdt:P31 ?instType. ?instType wdt:P279* ?class',
+    filterTypePattern: '?inst wdt:P31 ?instType. ?instType wdt:P279* ?class.',
     filterAdditionalRestriction: `FILTER ISIRI(?inst)
                         BIND(STR(?inst) as ?strInst)
                         FILTER exists {?inst ?someprop ?someobj}
@@ -664,10 +664,13 @@ const OwlRdfsSettingsOverride: Partial<SparqlDataProviderSettings> = {
     dataLabelProperty: 'rdfs:label',
     fullTextSearch: {
         prefix: '',
+        // ?score is a constant as regex() gives no relevance measure, but it is
+        // bound through an expression on ?search1 because some endpoints
+        // (e.g. Virtuoso) reject ORDER BY on a variable bound to a literal constant
         queryPattern:
         `?inst \${dataLabelProperty} ?search1
         FILTER regex(COALESCE(str(?search1)), "\${text}", "i")
-        BIND(0 as ?score)
+        BIND(if(bound(?search1), 0, 1) as ?score)
 `,
         extractLabel: true,
     },
@@ -722,23 +725,23 @@ const OwlRdfsSettingsOverride: Partial<SparqlDataProviderSettings> = {
         }
     `,
     linkTypesStatisticsQuery: `
-        SELECT ?link ?outCount ?inCount
+        SELECT (\${linkId} as ?link) (COALESCE(sum(?__out), 0) as ?outCount) (COALESCE(sum(?__in), 0) as ?inCount)
         WHERE {
             {
-                SELECT (\${linkId} as ?link) (count(?outObject) as ?outCount) WHERE {
+                SELECT (1 as ?__out) (0 as ?__in) WHERE {
                     \${linkConfigurationOut}
                     \${navigateElementFilterOut}
-                } LIMIT 101
-            } {
-                SELECT (\${linkId} as ?link) (count(?inObject) as ?inCount) WHERE {
+                }
+            } UNION {
+                SELECT (0 as ?__out) (1 as ?__in) WHERE {
                     \${linkConfigurationIn}
                     \${navigateElementFilterIn}
-                } LIMIT 101
+                }
             }
         }
     `,
     filterRefElementLinkPattern: '',
-    filterTypePattern: '?inst a ?instType. ?instType rdfs:subClassOf* ?class',
+    filterTypePattern: '?inst a ?instType. ?instType rdfs:subClassOf* ?class.',
     filterElementInfoPattern: `
         OPTIONAL {?inst rdf:type ?foundClass}
         BIND (coalesce(?foundClass, owl:Thing) as ?class)
@@ -820,7 +823,7 @@ const DBPediaOverride: Partial<SparqlDataProviderSettings> = {
         }
     `,
 
-    filterTypePattern: '?inst a ?instType. ?instType rdfs:subClassOf* ?class',
+    filterTypePattern: '?inst a ?instType. ?instType rdfs:subClassOf* ?class.',
     filterElementInfoPattern: `
         OPTIONAL {?inst rdf:type ?foundClass. FILTER (!contains(str(?foundClass), 'http://dbpedia.org/class/yago'))}
         BIND (coalesce(?foundClass, owl:Thing) as ?class)
diff --git a/src/data/sparql/sparqlModels.ts b/src/data/sparql/sparqlModels.ts
index 4686633e..249a5ea5 100644
--- a/src/data/sparql/sparqlModels.ts
+++ b/src/data/sparql/sparqlModels.ts
@@ -116,8 +116,10 @@ export interface LinkBinding {
 
 export interface LinkCountBinding {
     link: Rdf.NamedNode | Rdf.BlankNode;
-    inCount: Rdf.Literal;
-    outCount: Rdf.Literal;
+    /** May be unbound e.g. when an endpoint aggregates over an empty solution group. */
+    inCount?: Rdf.Literal;
+    /** May be unbound e.g. when an endpoint aggregates over an empty solution group. */
+    outCount?: Rdf.Literal;
 }
 
 export interface ConnectedLinkTypeBinding {
diff --git a/src/widgets/instancesSearch.tsx b/src/widgets/instancesSearch.tsx
index f880d6d9..9cbdf759 100644
--- a/src/widgets/instancesSearch.tsx
+++ b/src/widgets/instancesSearch.tsx
@@ -322,6 +322,7 @@ class InstancesSearchInner extends React.Component<InstancesSearchInnerProps, St
         
         const resultItems = this.state.items ?? [];
         const actionsAreHidden = this.state.querying || this.state.selection.size === 0;
+        const queryError = this.state.error ? tryFormatQueryError(this.state.error) : undefined;
 
         return <div
             className={cx(
@@ -344,6 +345,11 @@ class InstancesSearchInner extends React.Component<InstancesSearchInnerProps, St
             <ProgressBar state={progressState}
                 title={t.text('search_entities.query_progress.title')}
             />
+            {queryError ? (
+                <div className={`${CLASS_NAME}__error`}>
+                    {queryError}
+                </div>
+            ) : null}
             {/* specify resultId as key to reset scroll position when loaded new search results */}
             <div key={this.state.resultId}
                 className={`${CLASS_NAME}__rest reactodia-scrollable`}
@@ -672,6 +678,13 @@ function findEntityData(graph: DataGraphStructure, iri: ElementIri): ElementMode
     return undefined;
 }
 
+function tryFormatQueryError(error: unknown): string | undefined {
+    if (error instanceof Error) {
+        return error.message || error.name;
+    }
+    return undefined;
+}
+
 export function createRequest(criteria: SearchCriteria): DataProviderLookupParams {
     const {text, elementType, refElement, refElementLink, linkDirection} = criteria;
     return {
diff --git a/src/workspace.ts b/src/workspace.ts
index 66aefe2a..9bb029b2 100644
--- a/src/workspace.ts
+++ b/src/workspace.ts
@@ -64,7 +64,10 @@ export * from './data/rdf/rdfDataProvider';
  */
 export * as Rdf from './data/rdf/rdfModel';
 export * from './data/rdf/vocabulary';
-export * from './data/sparql/sparqlDataProvider';
+export {
+    SparqlDataProvider, type SparqlDataProviderOptions, type SparqlProviderChunkOptions,
+    type SparqlQueryFunction, SparqlResponseError,
+} from './data/sparql/sparqlDataProvider';
 export * from './data/sparql/sparqlDataProviderSettings';
 
 export {
diff --git a/styles/widgets/_instancesSearch.scss b/styles/widgets/_instancesSearch.scss
index a819df73..9c5691d7 100644
--- a/styles/widgets/_instancesSearch.scss
+++ b/styles/widgets/_instancesSearch.scss
@@ -96,6 +96,16 @@
     }
   }
 
+  &__error {
+    padding: 5px 10px;
+    color: theme.$color-danger;
+    font-size: smaller;
+    overflow-wrap: anywhere;
+    max-height: 8em;
+    overflow-y: auto;
+    user-select: text;
+  }
+
   &__rest {
     padding: 10px 10px 0 10px;
     border-top: 1px solid theme.$border-color-base;
diff --git a/test/data/sparql/sparqlProviderBasic.test.ts b/test/data/sparql/sparqlProviderBasic.test.ts
index 28180827..bb8cada9 100644
--- a/test/data/sparql/sparqlProviderBasic.test.ts
+++ b/test/data/sparql/sparqlProviderBasic.test.ts
@@ -4,7 +4,7 @@ import type {
     ElementIri, ElementModel, ElementTypeIri, ElementTypeModel, LinkTypeIri, LinkTypeModel,
     LinkModel, PropertyTypeIri, PropertyTypeModel,
 } from '../../../src/data/model';
-import type { DataProviderLookupItem } from '../../../src/data/dataProvider';
+import type { DataProviderLinkCount, DataProviderLookupItem } from '../../../src/data/dataProvider';
 import { type MemoryDataset } from '../../../src/data/rdf/memoryDataset';
 import * as Rdf from '../../../src/data/rdf/rdfModel';
 import { rdf, owl } from '../../../src/data/rdf/vocabulary';
@@ -93,6 +93,21 @@ describe('SparqlDataProvider', () => {
         );
     });
 
+    it('provides connectedLinkStats() with exact counts', async () => {
+        const provider = await makeSparqlDataProvider(
+            {},
+            {...OwlStatsSettings, filterOnlyLanguages: ['en']},
+        );
+        const stats = await provider.connectedLinkStats({
+            elementId: org.Organization,
+        });
+        expect(stats.find(s => s.id === rdfs.subClassOf)).toEqual({
+            id: rdfs.subClassOf,
+            inCount: 3,
+            outCount: 1,
+        } satisfies DataProviderLinkCount);
+    });
+
     it('provides propertyTypes()', async () => {
         const provider = await makeSparqlDataProvider(
             {},
@@ -228,6 +243,18 @@ describe('SparqlDataProvider', () => {
             ] satisfies DataProviderLookupItem[]
         );
     });
+
+    it('provides lookup() by type and text', async () => {
+        const provider = await makeSparqlDataProvider(
+            {},
+            {...OwlStatsSettings, filterOnlyLanguages: ['en']},
+        );
+        const items = await provider.lookup({
+            elementTypeId: owl.DatatypeProperty,
+            text: 'ident',
+        });
+        expect(items.map(item => item.element.id)).toEqual([org.identifier]);
+    });
 });
 
 function readPropertyValues(