The exact error message is:
Recently an error occurred while creating the CSS / JS minify cache: A group configuration for "include-footer" was not set.
I turned on the debug info, but that doesn't help. It only shows a list of files that were minified.
I searched for the error message, and found out that it is an old issue that should already be solved.
I switched on and off user agent groups and referrer groups. Could that be the problem?