What could cause commenting to suddenly not work for a particular channel if no changes to the configuration has been made? The person trying to comment gets the message "You are not authorized to perform this action". The EE version is 2.9.0 and there's been no upgrade between now and the last successful comment.
The "You are not authorized to perform this action" error can occur under these two situations (in addition to other situations too):
When EE determines that a posted comment is a duplicate. So if you keep posting a comment with "test" as the copy, EE will trigger the error.
When a person submits a comment and then uses their back button to post a second comment.