Tip Of The Day (3) : Linux Mint 13 and Petra Backports

Can't log in after the update of your Linux Mint with Petra Backports ?

Don't forget to read the other tips:

Tip Of The Day (5) : Batch Script to resize pictures in linux (Nemo)

Tip Of The Day (4) : Bulk renaming files in linux

Tip Of The Day (2) : Batch resizing pictures in linux

Tip Of The Day (1) : Screenshot 

After the update i shot down my PC, and the following day i couldn't log in and got 

"There was an error loading the theme XXXXXX" 

And no way to log or anything

Here is the Workaround : 

1. Press ctrl+alt+F1 to switch to the console
2. Log in
3. Then type sudo killall mdm
4.  and startx
5. Cinnamon will start correctly (we've just bypassed the login window)

if not done yet, make sure mint-meta-cinnamon , mint-mdm-themes are installed, and ubuntu-system-adjustments is up-to-date.

==> you've got a somewhat working PC now, but you'll need to repeat the process at every boot

Now fixing the issue : 

When the update was performed it erased the potentially custom theme we were using so mdm couldn't launch anything at boot ...

• Once in Cinnamon trying to start mdm in order to change the theme fails.
• Open /etc/mdm/mdm.conf as root
• Search for  HTMLTheme= and GraphicalTheme=
• Change theses values to : 

HTMLTheme=Mint-XGraphicalTheme=linuxmint

• Save the file, and reboot
• Log in, open mdm and choose the theme you want (html or gdm/gtk)

==> You've now recovered a fully working Linux Mint 13 with the Petra backports ! 

Alternate fix [source] : 

• Delete your MDM configuration with the command: sudo rm -f /etc/mdm/mdm.conf
• Reset the MDM configuration with the command: sudo touch /etc/mdm/mdm.conf
• Reboot the computer