after a few hours trouble shooting. i found that primitive mobs has a problem with Just Enough Resources. after removing this mod from the server and the client all works fine. if this works to fix your problem please report back here so we know its not just me.

I would also like to know about this! Thanks for the info.

I can confirm that removing JER from my client fixes this problem. Seems to be okay in a local game, but when I setup a server, no one can successfully connect without removing JER, all get the baby spider error.