ISPConfig and Lets Encrypt - Option permanently switched of by itself

Hello,
I'm running the latest ISPconfig with all patches and LetsEncrypt with certbot version 0.31.0.
On my server I have a couple of installations, most of them with a bunch of alias domains.
One of this installations does not run with Lets Encrypt.
When I switch on Lets Encrypt in the configuration of the website, this option is switched off again, after one minute.
The certs are created properly, as I can evaluate.
I activated the ISPConfig log to debug mode and there, I get this:
"Let's Encrypt Cert file: does not exist."
I think, this is the main reason, why the options is switched off again and again. But why?
Out of desperation, I just deleted all directories and configurations concerning this specific website to start from scratch, but it didn't work.
In the renewal config of the website, there is a section called [[webroot_map]]. In all other installations, I can see entries of the domains and alias domains. But in this case, the section keeps empty.
Does somebody have an idea, what's going wrong here?
Thank you in advance!
Ralf