hi stefan im no pc expert but have seen on various forums and steam that when a patch or update comes out for assetto then it can cause problems with servers resulting in checksum mismatch etc the solution 99 times is the server is running a old version and when people who,s game as been updated it then says checksum failed but once the server is updated or restarted it wwill work fine i wish you luck best wishes for christmas and new year 2017