On my minecraft server, me and my friend will get a ticking entity crash from a zombie, anywhere from 15 minutes to several hours apart. It isn't hard to fix, since I can just use the cofh killall command to kill it but it's pretty irritating.
Heres the log: https://pastebin.com/gNzR51zZ
And these are the mods we're using:
Ars Magica 2