I recently ran into a signing key issue on an Ubuntu system running gitlab-ee from the official GitLab repos but didn't notice for a while so my normal automatic upgrades hadn't been working to keep it updated. This may have also been affecting other package upgrades... not sure yet.
Anyway, the first thing to solve was getting the latest repo key installed. Downloading and running the script mentioned in step 2 of https://docs.gitlab.com/install/package/ubuntu/?tab=Enterprise+Edition did the trick for the key.
Afterwards, I was faced with the issue of being multiple Minor revisions behind. This manifested as this error:
