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We're migrating some old 10.10 users on Gnome to 14.04 using Unity. When they log on, however, the lightdm greeter complains "Failed to start session." /var/log/lightdm/lightdm.log notes:

[+181.14s] DEBUG: Session pid=3566: User thales authorized
[+181.15s] DEBUG: Session pid=3566: Greeter requests default session
[+181.15s] DEBUG: Seat: Failed to find session configuration gnome-classic
[+181.15s] DEBUG: Seat: Can't find session 'gnome-classic'
[+181.15s] DEBUG: Session pid=3566: Greeter start authentication

A successful login looks like:

[+113.16s] DEBUG: Session pid=1957: User ntimkovich authorized
[+113.17s] DEBUG: Session pid=1957: Greeter requests default session
[+113.17s] DEBUG: Seat: Stopping greeter; display server will be re-used for user session
[+113.17s] DEBUG: Session pid=1957: Sending SIGTERM

Our migration is in an intermediate state, so I'd like to avoid obliterating profiles (e.g. rm -rf ~/.gnome*), and we're also trying to push people to just use one desktop (the stock Unity) for simplicity.

What is the specific folder/file that is causing the GUI to request 'gnome-classic' that I could perhaps temporarily hide? Or even better, is there a way to tell lightdm (or whatever does this) that if it "can't find session 'X'" to revert to Unity?

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The user's .Xauthority file is the culprit. Moving the file and logging in will revert to the system defaults.

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