I have a problem with the login manager of ubuntu 12.04, it does not have a selection to login another user; instead it only allows administrator and guest.
Can you help me with this problem?
Which architecture do you run Ubuntu on? This looks also like the bug https://bugs.launchpad.net/ubuntu/+source/accountsservice/+bug/952909?comments=all which affects accountsservice on some architectures like ARM (e.g., on Toshiba AC100) or PPC. It has already been fixed in the original Debian packages, but not yet in Ubuntu's packages AFAIK.
If that's the case, read also 12.04 User Accounts not listed in System Settings User Accounts, a question where the architecture has been explicitly stated.