Game Desyncs when playing with Friends + mods

Whenever I want to play a game with friends the game loads in and immediately starts saying that there is a Desync (Beat# 1) I don't know what the problem could be.

Everyone doesn't have the mods.

Or hasn't activated them

Or one of them has the mod but is an older version to what you are using

Thanks for all the replys

Please post a full Game.log next time, so we don't need to guess so much.