So what would be the reason for this user getting "Insufficient Permission" errors if they are part of the registered group which has sufficient permissions? All this user was attempting to do was post a reply. Hmmm.. I'll see what happens if I create a non-admin account on my end.
It's worth checking the permission analyzer for the specific case you're testing to see the elements at play (and confirm they're working as you expect). It may also help show if there's an add-on interfering (such as by preventing permission rebuilds, which has happened).