I wish I could help, but I have no idea what might be causing a extra user account to be messing up your system. Are they all Admin users, or might some be standard?
Is there anything about the User Name that might be causing problems?
I have not heard of a limit on the number of User Accounts, but do you have plenty of space on the hard drive? If not maybe some other problem is keeping the system from logging into the new account. I assume you can still log into the other accounts?
Have you looked at the lusrmgr.msc dialog to see if anything shows up there?