I wanted to join a LanServer but when logging in cames the game crash withthe erro: unexpected error.
The Crashreport is: The game crashed whilst unexpected error
The game crashed whilst unexpected error
Error: java.lang.NullPointerException: Unexpected error

What mods are installed on the hosting client? And what mods are installed on the joining client? Are you just using BigReactors or a modpack or did you modded the client yourself? Also a full crash report would be better than just a few lines.