Was it frequently in a short period of time? Was it only when trying to do anything specific?
I can't really find out what caused that because these kinds of errors aren't logged (could be the database server was under maintenance by the host, malfunctioning, or there's a version incompatibility somewhere), but I'll try to add logging for this, and hopefully find out more!
Edit: additional logging of database errors is now in place, let's see if it happens again.