I’m not sure if this was because of the artifacts.chirpstack.io server certificate or the certificate used for signing the .deb packages.
In case of the server certificate:
The server certificate is issued by Letsencrypt. Their root certificate expired a few months ago. This caused for some users an issue, because the new root certificate was not yet installed on their system. If this is the case, then you need to update the CA certificate bundle on your machine. See also: Chirpstack repository: The certificate is NOT trusted. The certificate issuer is unknown - #4 by brocaar
In case of the .deb signing certificate:
This certificate was renewed beginning this year. You might need to re-fetch it:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1CE2AFD36DBCCA00