Indeed, this time I switched to Amsterdam EHAM Schiphol, then it's working again, then I switched back to Frankfurt EDDF and then the AFCAD message appeared again?
This means, from your position, GSX couldn't find a valid AFCAD for EDDF. You don't say which EDDF you are referring to, so we can only guess. The file might have been rejected for some reason.
You can try enabling Logging in the Troubleshooting section of the GSX Settings, and choose "Restart and rebuild the airport cache" in the Couatl menu.
After the cache has been rebuilt, exit the sim and check the log file at %APPDATA%\Virtuali\Couatl.LOG, if it says some files has been rejected and why.
Please read this thread, and follow the suggestions there, to see other reasons why an AFCAD might be rejected:
http://www.fsdreamteam.com/forum/index.php/topic,22516.msg152017.html#msg152017