How to fix "The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016)" on 20.04 LTS
Solution 1:
The problem seems to have been fixed - after running sudo apt install --reinstall ubuntu-dbgsym-keyring the problem has went away.
Solution 2:
The solution offered at https://bugs.launchpad.net/ubuntu/+source/ubuntu-keyring/+bug/1920640/comments/32 worked for me:
sudo apt remove --purge ubuntu-dbgsym-keyring
sudo apt-key del 0xC8CAB6595FDFF622
sudo apt install ubuntu-dbgsym-keyring