What this short tutorial is going to show you to do is how to disable Lenient Kick from your server. Lenient kick will kick any client that is running an older client than the server host. The way to disable this is actually really easy.
Go into your server directory, and go to Scripts\Misc\ClientVerification.cs. Then within that file, search for:
Code:
private static OldClientResponse m_OldClientResponse = OldClientResponse.LenientKick;
and change it to:
Code:
private static OldClientResponse m_OldClientResponse = OldClientResponse.Ignore;
Save and restart your server and the problem is fixed.