have you manually deleted any users from the database?
I'm not sure if this could be the cause though.
otherwise I'm out of ideas..
try turning on debug logging and see if the logfile tells something.
If you have a problem, give us enough info of what you have done, what is configured in relation to your question.
Enable debugging and post it's censored but relevant info. Don't forget to write which version of SiT you're running.