In order to see the full context of an exception the following setting $wgShowExceptionDetails = true; should be enabled in your LocalSettings.php file.

 Has description
SPARQLStore/Exception/BadHttpDatabaseResponseExceptionIn cases where the response from a triple-store could not be interpret a BadHttpDatabaseResponseException is raised
SPARQLStore/Exception/HttpDatabaseConnectionExceptionA connection to an external endpoint can be severed caused by network lag or any other form of data loss