Please check the application rule. You might be having a certain stale class rule which once existed when you added that rule to the case&data tab of the application but then you have removed that case type. This change didn't reflect on the class rule. So during a new casetype addition to the Application rule, it validated the existing class rules / case type and giving out an exception saying invalid class name.
Find the stale class rule and remove the rule. Resave the application rule once and then try to add the new case type to the applicaiton.