If you encounter an error message similar to the following:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://archive.serverdensity.com/ubuntu all InRelease: The following signatures were invalid: KEYEXPIRED 1509014164 KEYEXPIRED 1509014164 KEYEXPIRED 1509014164
This is due to the key that we use to sign our agent packages with expiring.
How to Fix
To fix this issue you need to update the key on your server to the new key that we use to sign our packages. You can do this with the following:
curl -Ls https://archive.serverdensity.com/sd-packaging-public.key | sudo apt-key add -