@fergru have you compared that particular renewal.conf file against the others to see if you can find a difference? Is your renewal urgent right now?
A very quick fix right now to get it renewed is to do the following, note this is only a quick fix solution.
# renew_before_expiry = 30 days
version = 0.19.0
archive_dir = /etc/letsencrypt/archive/t2792.greatnet.de
cert = /etc/letsencrypt/live/t2792.greatnet.de/cert.pem
privkey = /etc/letsencrypt/live/t2792.greatnet.de/privkey.pem
chain = /etc/letsencrypt/live/t2792.greatnet.de/chain.pem
fullchain = /etc/letsencrypt/live/t2792.greatnet.de/fullchain.pem
# Options used in the renewal process
[renewalparams]
account = xyxyxyxyxyxyxyxyxyxyxyxyxyx
authenticator = standalone
installer = None
#[[webroot_map]]
#t2792.greatnet.de = /srv/www/htdocs
#www.t2792.greatnet.de = /srv/www/htdocs
then manually stop your web server, and run certbot-auto and see if it renews for you and if so then just restart the web server afterwards.