I guess it's a directory permissions issue. Mostly the /files/cache directory.

hmm... I empty all the files in the directory /files/cache and cleared my browser cache and still have the same trouble.
The only thing that has worked for me is if I'm still logged as the Admin, I can go ahead and create the other users. The only problem with this is, I cannot create another super user such as Admin. I've create another user who is placed in the admin group. But once I updated all the users who were in the users listed were disable. And once I log out as the Adminl, I am not able to log back in again. Thanks for the help.

Thanks that worked. I just now have to send out a mass email letting the other users that they need to do the same. Thanks for the help.

This could maybe have been fixed by copying the password salt from the old config/site.php to the new file.

I get this probelm off-and-on regularly. I thought I'd solved it by telling everyone to log in after visiting the page including the www. But now that doesn't even work. How are you supposed to clear the cache when you can't log in ad admin? This occurrence of the log-in problem just started happening recently (within the last week or two) and I have not made any changes to the website whatsoever!