Not sure what exactly causes this but had a player connect today and it disconnected him. In the console it said Network Disconnect: A fatal error has occurred, this connection is terminated. He was able to reconnect the next time he tried though.

Here is the error from the console: https://gist.github.com/infamyreloaded/4f1685d61b36e2dc5cfa111504c81de5
SpongeForge
Minecraft: 1.12.2
SpongeAPI: 7.1.0-70763aa8
Sponge: 1.12.2-7.1.4
SpongeForge: 1.12.2-2768-7.1.4
Minecraft Forge: 14.23.5.2807
primitivemobs-1.2.3a