I haven't touched gs3d multiplayer the last couple months, but I think it has something to do with invalid pointers.

I think EpsiloN's idea in this thread ( http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=450296&page=2 ) of doing only local bullets is a way to completely solve it.

What at that time somewhat helped for me was to wait a bit before doing ent_remove (I made the bullet invisible, break the while loop and removing it from groups etc.) which seemed to help quite a bit, but not solving it completely.
It is playable though when warn level is set to a low number, though this is ofcourse not recommended if your going to make a commercial game of it since sooner or later some clients will get weirds bugs .
But for short/quick games its fine.