I have a real issue with many of my users. The users can be seen in admin panel like this:
But when I try to reveal their email address, It wont show anything to me. And also their public profile gives a 404 error.
As far as I see, all of these users are almost inactive (they are active in terms of activation but inactive in terms of activity in forum). So I think maybe its been caused by a broken automatic inactive user removal in the very long past.