Some more datapoints:
in /var/syslog I find:
Oct 22 06:33:56 mail certbot[34963]: Failed to renew certificate mail.kukulies.org with error: The requested apache plugin does not appear to be installed
In this forum I found:
about this issue, but neither the apt package can be found nor the cited link works (Error 404)
root@mail:/var/log# certbot plugins
Saving debug log to /var/log/letsencrypt/letsencrypt.log
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* standalone
Description: Spin up a temporary webserver
Interfaces: Authenticator, Plugin
Entry point: standalone = certbot._internal.plugins.standalone:Authenticator
* webroot
Description: Place files in webroot directory
Interfaces: Authenticator, Plugin
Entry point: webroot = certbot._internal.plugins.webroot:Authenticator
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root@mail:/var/log#
To answer your question about system timers:
Mon 2023-10-23 19:08:37 CEST 8h left Mon 2023-10-23 09:27:36 CEST 49min ago certbot.timer certbot.service
Meanwhile I got into the right direction. My apache2 had start problems. Some modules were missing. Mainly apt install python3-certbot-apache and some others. (I neglected apache2 a bit, since it was not primarily the goal to run a web server. mail server is the primary task)