Since recently, orbi and letterbox are failing to run apt update because the apt gpg key for the bytemark repositories has expired. I guess we need to find the new key in the bytemark documentation and import it.
Note this doesn't prevent installing updated packages, as long as they come from the regular Ubuntu repositories.
root@orbi ~ # apt update Ign:1 http://repo.bytemark.co.uk/3ware-tools/xenial ./ InRelease Ign:2 http://repo.bytemark.co.uk/hp-raid/xenial ./ InRelease Hit:3 http://repo.bytemark.co.uk/3ware-tools/xenial ./ Release Hit:4 http://mirror.bytemark.co.uk/ubuntu xenial InRelease Hit:5 http://mirror.bytemark.co.uk/ubuntu xenial-updates InRelease Hit:6 http://mirror.bytemark.co.uk/ubuntu xenial-backports InRelease Hit:7 http://repo.bytemark.co.uk/hp-raid/xenial ./ Release Hit:8 http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease Err:9 http://repo.bytemark.co.uk/3ware-tools/xenial ./ Release.gpg The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 Get:10 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB] Hit:11 https://repos.influxdata.com/ubuntu xenial InRelease Err:12 http://repo.bytemark.co.uk/hp-raid/xenial ./ Release.gpg The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 Get:13 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 Packages [428 kB] Get:14 http://security.ubuntu.com/ubuntu xenial-security/universe i386 Packages [373 kB] Fetched 910 kB in 1s (736 kB/s) Reading package lists... Done Building dependency tree Reading state information... Done 1 package can be upgraded. Run 'apt list --upgradable' to see it. W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.bytemark.co.uk/3ware-tools/xenial ./ Release: The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.bytemark.co.uk/hp-raid/xenial ./ Release: The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 W: Failed to fetch http://repo.bytemark.co.uk/3ware-tools/xenial/./Release.gpg The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 W: Failed to fetch http://repo.bytemark.co.uk/hp-raid/xenial/./Release.gpg The following signatures were invalid: KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 KEYEXPIRED 1551093651 W: Some index files failed to download. They have been ignored, or old ones used instead.