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Checksum mismatch on the server. What to do?
What is a checksum mismatch? A checksum mismatch is what happens when a file on the client and on the server are not identical.

When players get this error it can be for various reasons, but most commonly they are:

• A recent Assetto Corsa update has not been applied to either the client or server. As such the files on the client and server don't match.
• Some (add-on) content on the server is not the same as on the client.
  
  

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.

In the past we’ve experienced that Kunos Simulazioni forgot to release an update for both the server and client at the same time.

If that is the case than there isn’t much we can do except for waiting a few hours and trying again. Aditionally you could check the Assetto Corsa forum: http://www.assettocorsa.net/forum/index.php
To see if other users are experiencing the same problem and to bring it to the attention of the developers.

Server and client are up-to-date, but I still get a checksum Mismatch error. What should I do?

The best way to find the content that is causing the Checksum Mismatch is by looking at the web console in the control panel when joining a server.
It should indicate which file is generating the checksum error. You should then update this file on the server (or on the local clients that can’t connect) so that both the clients and server use the same file.

For example, the mercedes_sls is known to cause checksum mismatch errors.
To resolve this, go into your Assetto Corsa clients installation directory. Browse to the content > cars > mercedes_sls directory, and the upload the data.acd file from here to your server. Replace the existing file on the server.

Deprecated information:

If you have used the exact same cars in a different track and they work without any issue:

1. Go to your server's files, and go to content > tracks > the track you've on the server.
2. Now that you're looking in the desired track folder, you will see a folder called "data"
3. Click on the "data" folder to open it and view its content. There should be a file called "surfaces.ini".
4. Delete the file surfaces.ini located in the folder and upload the one located at your clients game's folder. (found in the exact same location)

If you have used the exact same track with other cars and they work without any issue:

1. Go to your server's files and go to content > cars > the folder of each car you've on the server.
2. Once you're on the desired car's folder, you will see a file called "data.acd"
3. Delete the file data.acd located in the folder and upload the one located at your game's folder in the exact same location.
   
   BE AWARE: Some cars may not have any data.acd file in its folder. If you experience a checksum mismatch with a car which has no data.acd file in the folder, reupload the car and try again.