When running a query or synchronizing with JIRA, JIRA Client shows an error like
Cannot run query (Cannot understand the remote server (Error on line 1: Content is not allowed in prolog.))
The "content not allowed in prolog" is actually an error from XML parser. It means that an XML document was expected, but something else has been received.
This effect can be caused by a number of potential problems. Please look for a suitable solution in the articles listed below, or write to support if no solution can be found.