Graphdb sparql cannot make federated query - sparql

Question

I am trying to make a federated query using my local GraphDB database and SPARQL endpoint. For some reason I always get errors or empty columns in my result table.
I tried several SPARQL endpoints which are working perfectly fine on FactForge. My code and error message are below can you help me to get it working.
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX bif: <bif:>
SELECT ?duration1 ?duration2{
VALUES (?start ?end)
{("2011-02-02T14:45:14"^^xsd:dateTime "2011-02-04T14:45:13"^^xsd:dateTime)}
SERVICE <http://dbpedia.org/sparql> #-- Virtuoso
{ BIND ((?end - ?start)/86400.0 AS ?duration1) }
}
Error 500: error
Query evaluation error: org.eclipse.rdf4j.query.QueryEvaluationException: Virtuoso 22023 Error Can not load own service description
I am using the free version of GraphDB. My OS is macOS Mojave 10.14.5. I directly installed GraphDB from the website and i never used RDF4J before. Also after this error i installed a local Virtuoso SPARQL endpoint and i can use federated query from that endpoint using my local GraphDB as a service. But i cannot use federated query from my local GraphDB endpoint using my local Virtuoso endpoint as a service since it gives the same error above.

Answer 1

The root cause of the above error is "Virtuoso 22023 Error Can not load own service description", which is handled as an unknown runtime exception resulting HTTP Code 500.
I suspect this was a temporal glitch in the DBPedia service since the same query currently works from GraphDB tested with the latest GraphDB 9.0 versions and an older GraphDB 8.x version via FactForge service.