No permission error for new members

Received a report this morning from a new user:
it doesn't log me out unless I select "logout", I still see my name in the corner and everything, but it won't let me load any pages, not even the front page, I get the "no permission" notice and the login box

What could be causing this problem? I disabled all recently added add-ons and used a test account, but the permission still popped up.

We dont use the add-on called moderator extensions and never have.