A friend brought me a Ubuntu system with the following issue:
On entering gdm, the login prompt was displayed on a black background and the following message appeared in the top-right corner:
“The configuration defaults for GNOME Power Manager have not been installed correctly. Please contact your computer administrator.”
Trying to log in as any user account was unsuccessful.
I don’t remember where I found the fix, but here’s what I did to fix it:
Once the gdm login screen appears, press CTRL-ALT-F1, which will switch you to a text-based login prompt, and log in using an account that has sudo privleges.
Run the following command:
sudo dpkg --configure -a
It may take a minute or two to complete. After that:
After the reboot, when gdm appears you should have your colored background and the message should not appear.
Questions, comments, and feedback on this are welcome.