How I fixed the "fatal error" that kept happening for me.

I'm making a zombie modpack for me and my friends to play, but the server keeps giving me and error saying "A fatal error has occured connection lost" or something like that. I've checked other threads but the issue seems different in each one with different. I'll attach my log.
https://mclo.gs/1SEfFiU

The pack works fine singleplayer but when I join the server it happens. Creating a new world on the server itself doesn't do anything both before and after generation of a new world.

I seem to have fixed the fatal error somehow; but now the screen is stuck on logging in.
EDIT: Nevermind, its back.

Never mind again, I fixed it by removing something. Don't know what, but whatever. Too future people, it fixed when I went through the mods and removed mods that were client only that for some reason can be downloaded on the server. Also, if your using any type of custom world gen, that might of been what was. I had the world gen set to the custom thing, I changed it after removing client mods, so I don't really know; Hope this was useful.