Server and client are up-to-date, but I still get a checksum Mismatch error. To see if other users are experiencing the same problem and to bring it to the attention of the developers. Aditionally you could check the Assetto Corsa forum: If that is the case than there isn’t much we can do except for waiting a few hours and trying again.
In the past we’ve experienced that Kunos Simulazioni forgot to release an update for both the server and client at the same time. The first issue is quite easy to fix by using the steam update function in the control panel and using the verify game cache files in Steam on the client. As such the files on the client and server don't match. When players get this error it can be for various reasons, but most commonly they are: What is a checksum mismatch? A checksum mismatch is what happens when a file on the client and on the server are not identical.