Sometimes, there are notifications, like this one:
Failed to new certs of xxx: Unexpected status: invalid. Renewal will be retried in 12 hours
These often require extra context to understand what went wrong, or how to rectify the problem (for many backup failures, for example).
It would be great if the notification provided some sort of a pointer to jump into Cloudron's logs at the time of the event in this case, to dig into the problem further.
Right, we are actually fixing notification for the next release (6.3). In the meantime, Domains -> Renew all certs and then check the logs (button on the same page) if it fails. Usually, Let's Encrypt errors are temporary and currently Cloudron is a bit overzealous reporting those errors.