System settings won't open on Ubuntu 20.04 LTS after adding System76/pop PPA

Solution 1:

Finally I found a solution!
The cause of this problem was pop-theme, that I installed once, plus its repository entry, that provided me the broken System settings (gnome-control-center). In order to solve this:

1. I installed Ubuntu desktop PPA.

   sudo add-apt-repository ppa:ubuntu-desktop/ppa
   

2. I removed all System76/pop (and other trush) entries, with update-manager, leaving Ubuntu desktop the only one active.

3. Now that I have the proper repository entry added, I re-install gnome-control-center.

   sudo apt install --reinstall gnome-control-center
   

4. Enjoy Gnome System settings!

Solution 2:

I had the same issue. I was able to fix by removing the GNOME extension/theme which caused the issue and then running:

sudo apt install --reinstall gnome-control-center

Solution 3:

sudo apt install ppa-purge

For the next step the PPA has to be active.

sudo ppa-purge ppa:system76/pop

This removes packages from this PPA if they are not available from Ubuntu repositories, or reverts to Ubuntu package versions.

Solution 4:

Hmm... a variant of this:

• Ubuntu 20.04.1 LTS
• No Pop!_OS' PPA - present.
• sudo gnome-control-center # worked, from a terminal/shell,
  but not so without 'sudo', no GLib-ERROR, with no output at all.
• "Ubuntu software" - also affected, didn't try 'sudo' there though.
• dconf reset -f /org/gnome/control-center/ didn't appear to help.
• Neither did the apt --reinstall below.

Remedy:

• Reboot into "recovery console"
  hit ESC ONCE after the BIOS displays to get to the boot menu)

• run the "dpkg" option to install / fix packages
  this brought in a gcc1 -thingie, didn't pay attention to exactly what*

• also run fsck option

• enable networking

• drop to root prompt,
  press ENTER one extra time at the extra prompt

• run:

$ apt-get install -f --reinstall gnome-control-center gnome-control-center-data
$ reboot

Note:
Remedy worked only as it was run in recovery mode.