I have had this issue with Drupal forum, specifically with Advanced Forum module & Forum access module. I keep getting this access denied: you are not authorized to access this page whenever i'm accessing one of my forum section IF i'm not login as an administrator (user id #1). My guess would be a permission problem either on forum access module and/or user permission depicted as below:

From the above, both anonymous user & authenticated user should be able to view the forum, but the fact is the contrary. So, why only the user id #1 can access the forum while other cannot, that's odd. The main problem lies on the bug within forum access module.

After ticked off the anonymous user and press save, then tick on anonymous user again and press save. Both anonymous & authenticated user can access the forum without any permission issue. My conclusion would be the forum access permission isn't being saved properly. Therefore, i posted this article as my personal reference in case such event occured again, so next time i won't get a headache.