I made a server yesterday for some friends and I. I only installed Create, no other mods. Every time we tried to join, it would give us an unknown recipe serializer error. It would say things like create:milling, create:splashing, create:block_zapper. We aren't able to connect to the world at all. I can't find anything about it online either.

Create is required on both the server and client to work. Make sure to launch the forge-xx.jar and not the minecraft-xx.jar when launching the server

This solved it, thanks!