If you get to a GUI login screen and/or can login to a desktop session as guest, then I think what that's telling you is it's something specific about YOUR (i.e. admin user's) X session startup

Some common problems are (1) your user's home directory not writeable (because of permissions/ownership or disk space - I think we can count out disk space since the guest session starts OK, presumably that's on the same partition) (2) the user's ~/.Xauthority (or more rarely ~/.ICEauthority) files being unwriteable because of permissions/ownership (3) something else in that particular user's startup (possibly in an non-default .xsession or .xinitrc file or a user-specific desktop application / service)

If you can get access to the user's home dir (either by logging in normally in a virtual terminal, or from the root console) you can try to find the problem e.g.

Code:
ls -ld /home/user

ls -l /home/user/.{ICE,X}authority

less /home/user/.xsession-errors