I used the Test Permission tool and confirm that all newly registered members after a certain point yesterday all received "You do not have permission to view this page or perform this action." message on ALL pages.
I thought of an addon conflict, however, disable addons one by one does not seem to fix it.
What I did was click on each member, assign them to a secondary usergroup and then changed it back it registered usergroup.
Waking up this morning, I got the notification by another member that it's happening again and those I thought I fixed yesterday still have the problem as well as all new members since then.
My guess is the permission table somehow got corrupted after a specific time point.
Is there anything I can do to fix it?