Hi Osiris,
Thank you for your time!
The certificate was first issued on the 21st of November 2021. The renewal took place at 20th of January. All seemed good, but the server kept using the old (still active) certificate. So according to acme I have a valid certificate, but the server is not using it.
That is why I tried a forced renewal.
The content of the backup directory is:
-rw-r--r-- 1 root wheel 0 Feb 12 15:11 cert.bak
-rw-r--r-- 1 root wheel 0 Feb 12 15:11 fullchain.bak
-rw-r--r-- 1 root wheel 0 Feb 12 15:11 key.bak
All empty files.