I found this issue, because I got notifications from the Let's Encrypt Expiry Bot.
So I checked all logs and didn't see any errors. Froxlor was showing me that a lot of certificates needs to be renewed.
Afterwards I ran the master cronjob with --force and --debug. Also no errors and I saw messages like "[information] Updated Let's Encrypt certificate for example.tld".
So this was looking also good, but no certificate was updated.
I checked acme.sh --list and saw that no certificate were updated, too.
So I came to the point that acme.sh was not being executed at all.