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I had the same problem using XOOPS 2.0.3 a couple months ago. I thought I had been hacked, as I also lost all my users names and passwords.
So, I erased the entire site, set up a fresh admin name and password, installed everything again, erased the admins name from admin.php and index.php in the news module and made everything visible for anonymous users.
It's working now.
Daniel