I get into this endless login-cycle if I attempt logging with my main user - I tried Gnome Classic and regular Gnome, but after I login it sends me back to the login.The only way I can login is through another user.This means that there's some hidden setting file in the home directory that messes things up.How can I debug this further?  
Upgraded to Gnome 3.4 on Debian, can't login to my main user?
debian;gnome
Since it only happens for a specific user, examining this user's ~/.xsession-errors could be useful.Since you're using Debian, have a look at the files in /etc/X11/Xsession.d, they're sourced by Xsession and thus give you an idea of what's happening when a new X11 session is started; e.g. 40x11-common_xsessionrc is the place where ~/.xsessionrc (i.e. USERXSESSIONRC set in /etc/X11/Xsession) is sourced.