Upgraded to Gnome 3.4 on Debian, can't login to my main user?

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http://unix.stackexchange.com – 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? (HowTos)