I literally cannot find a single thing about this particular bug, but when a player attempts to go to the nether they're instantly kicked off the server and left with a wonky "fatal error..." disconnect message. The problematic entity is EnderIO: 5, but what actually is that entity exactly...?

• EnderIO: 2.3.0.429
• EnderCore: 0.2.0.36
• Minecraft: 1.7.10
• Forge: 7.10.99.99

http://pastebin.com/Nnz3DGrk

Probably configuration mismatch between client and server.

Which was the first thing I checked for, however I haven't found a difference between the files (yet)...

edit; P4Diff says the files are identical, both the EnderIO and endercore files...

Well I eventually got passed it by disabling 'replaceWitherSkeletons' on just the server, so it's something to do with that...

This was also happening to me, same versions. My friend was in the nether, and couldn't rejoin after randomly getting kicked.