AH… is this an upgrade from a pre-V5 version? If so we have had a small handful of users whose permissions got scrambled – something we have been unable to replicate I am afraid even when an affected user gave us a backup of their pre-upgraded database to try it with. very sorry of this has also happened to you.
Are you permissions complex? They can be fixed manually but the safest and simplest method is to use the permission Reset option on the admin menu but this means that the user-group/permission assignments have to be re-applied to your forum records as these will be reset also – plus the permissions will be reset to their original defaults.
Let me know which way you prefer to go and if, indeed, this is the root of the problem.