I am getting the following error when Categorize questions functionality is enabled:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, postmaster@dummy-host2.localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

I have installed the Domain Access module on this site. When I tryed FAQ module on a test site with Domain Access turned off, it worked perfectly. Thereforoe I assume that categorization is in same way conflicting with Domain Access module.
The basic FAQ functionality (FAQ questions) works without issues also with domain Access, only categorization does not.

Any ideas how to solve this please?