"File Mismatch" bug after mod update. This bug happens when a player has an out-of-date version of a mod installed and attempts to join a server using the newer version of the mod. They will be prompted to download the update via in-game but will continue to see a 'file mismatch' message when they attempt to join. For this one, the player has to unsubscribe and resubscribe to the mod via the Steam workshop or verify their game files via Steam. The 'File mismatch' error can also be displayed legitimately when a player has a newer version of a mod but the server is running an older version, in this case, the server requires updating and restarting.

You can try to verify integrity of game files through steam to fix it on your side (client).