I had something like this occur as well a few months back. One of our members mysteriously vanished from the database for some unknown reason (the database has never been altered in any way either, nor had any tables been repaired up until that point). Luckily it doesn't occur very often.
Perhaps merely optimizing it could occassionally cause this, not entirely sure.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.
