Is there anything in your BI Server log (nqserver.log) to suggest that it crashed?
Do you have any core dump files present?
Nothing to suggest that it has crashed (we see those too often also but generally the two aren't linked). When it occurs all processes are running ok and it's often not for every user that the issue occurs but generally it's either for just one or two or sometimes everybody at once. I had thought that it would be a session expiry time type issue but have been unable to reproduce the issue even when setting those to be very short.