There is an issue with the certificate associated with the cellar of our hds zone, we are investigating.
12:58 UTC: The issue is resolved.
Here is what we know so far:
The revocation server of the Certification Authority providing this certificate says that this certificate has been revoked on 2021-06-23, except it was still accepted just fine a few hours ago.
We have asked for a reissue of the certificate (this is an automatic operation). The reissued certificate has been installed and is working fine. Meanwhile, we have asked the CA about this revocation without any warning or notice and are waiting for an answer.