I was fooling around with my install of Coppermine and started down the bridging path, even though I have no software to bridge to.
When I backed out of the bridge dialog, I discovered that my Groups had reverted to their original state - a group I had created disappeared. Unfortunately, I had assigned three user accounts to that group. When I looked at my user list, those people no longer showed up. I tried re-creating that group, but still could not see the users.
Meanwhile, they were able to log in to Coppermine, but no longer had upload privileges.
When I used mySQL to look at the database, I discovered what had happened. The originally-created group had an ID of 5. The re-created group had an ID of 8 (I had done some more experimenting in the meantime). The "missing" users were still assigned to Group 5, but there was no Group 5.
In mySQL I edited the "missing" users and assigned them to Group 8. Voila! They reappeared in the Users listing.
This seems like a bug to me... ???
I had the same problem, thanks for the guide. Now i have fixed it.
:o