You can set the "TraceOn" param in the "[Parameters]" section of the Virtuoso configuration file such that it is enabled on server startup
as detailed at:

http://docs.openlinksw.com/virtuoso/databaseadmsrv.html#ex_traceoniniopt

[Parameters]
....
TraceOn = soap, errors
...

This will enable logging of additional information regarding SOAP calls and SQL run-time errors into the virtuoso.log file.