It appears to be related to being near a zombie. For the first crash, it occurred during a fight with a zombie, after I believe I landed the killing blow. Next three crasheS occurred immediately upon entering the server, I would guess because the zombie or its body was near me. The fourth crash occurred a few minutes later, after I had left the aggro range of another (possibly the same zombie). Crash number 5 occurred when I tried to log in again, and was immediate.