You're killing your own game
@a-slice-of-pizza, have you changed your Config file in the mud runner directory or added stuff to your media zip? edited something? you can try running spintires mod game files check (activated) and that pop up will dissapear dont touch the spintires mud runner directory in steam or the one in %appdata% to avoid that pop up, only spin mod can touch it without setting that pop up on
another thing would be have you changed any of the scrips in your media zip? say like edited the fuel amount on an addon, a vehicle itself or something? this gives me warning like you got. just a thought
Also, if you're using the ModLoader you have to manually check the box to disable gamefile check for multiplayer..