Sorry if this is something that's been said before, I've had a look around but can't find anything, first, where on earth do you find the API key, I cannot find it anywhere. When exporting config.php from xbilling I also get a 500 server error, with error reporting on I just get a blank page
To find your API key, log into phpmyadmin as root and navigate to sentora > x_settings row 28.
Or you can use the GateKeeper module found here:
http://zppy-repo.mach-hosting.com/
It can be installed using RepoBrowser module, command line, or manually uploaded through your panel in Module Admin.
About the 500 error, I am not sure. Will have to check your config.php and make sure your panel and xbilling frontend URLs are correct.
When do you get the 500 error? What are you clicking on just before it?