Could you provide more details on the error you’re seeing?
While Let’s Encrypt will be perfectly happy to issue certificates for IDNs, it’s not uncommon to run into compatibility issues while trying to actually use the domain, depending on the software. It’s possible that Plesk’s Let’s Encrypt implementation does not support IDNA 2008, and that’s causing this problem. IDN also tends to not work with some email clients, for example.
Similarly, Chrome doesn’t support IDNA 2008 either, which would cause anyone typing ziegenfuß.de into their browser to get “redirected” to ziegenfuss.de instead, though direct links should work. This is probably not optimal if you don’t own both domains.
(I’m not entirely sure if ß -> ss would fall under some unicode normalization rule that Let’s Encrypt would have to enforce on their end as well. Either way, the other compatibility issues are probably a bigger problem.)
Do you know if Plesk asks for the domain in punycode notation? I did a quick test using the punycode version of your domain (xn--ziegenfu-xya.de) and can confirm that Let’s Encrypt handles it correctly. requesting the challenge file from http://xn--ziegenfu-xya.de/.well-known/acme-challenge/. Perhaps entering the domain in that notation will work. If that doesn’t work either, I’m afraid that’s a bug in Plesk’s Let’s Encrypt implementation - you’d have to talk to their support.
I would personally still avoid this particular domain due to the other compatibility issues, unless this is mostly for personal/internal use.