Before reporting this as a bug I am asking the community to check whether they are seeing the same issue in their Plesk:
When you create a domain like "große-anzahl.xx" and then try to add another domain "grosse-anzahl.xx" which is a different spelling, the second domain is denied, because Plesk argues, that the domain already exists. It seems that in the case of using "ß" in a domain name Plesk transcribes that internally to "ss" (which is the correct transcription but should not be done for domain names), then compares the transcribed version to the other domain and sees equality.
When you create a domain like "große-anzahl.xx" and then try to add another domain "grosse-anzahl.xx" which is a different spelling, the second domain is denied, because Plesk argues, that the domain already exists. It seems that in the case of using "ß" in a domain name Plesk transcribes that internally to "ss" (which is the correct transcription but should not be done for domain names), then compares the transcribed version to the other domain and sees equality.