After far, far too long, I have finally found the problem.
Your registration integration is broken. It's allowing duplicate usernames.
UBB will find the first instance of a username and use it, as it knows, for a fact, that there can not be duplicates.
You will need to delete all the dupe users, then patch the system to no longer accept usernames if there's already an existing name in a different case.
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Charles Capps
Programmer, Infopop Corporation
Please do not contact me privately for support - post on the board or open a support ticket instead!