Yet Another Permission-Denied Bug Report.
I didn't see this particular combination of circumstances in any of the other reports, but if I search for unread posts ala http://www.epicski.com/forum.php?action=newposts, the unread-posts list contains the following entry, which is in a restricted Post Reports and Infraction Notices forum to which I don't have access:
And clicking on the link brings up the permission-denied screen for me.
Maybe this is just a one-off, something's-been-changed-in-midstream type of quirk; if so, just ignore this bug report. At worst, action=newposts may be returning posts that some users are not privileged to view.