Hi all!
today I have a nearly 2h call with the Apple support. We figured out, that the sync work if I start my Mac in Secure Mode (start with pressed Shift key). The conclution was that some application block the connection.
After the call I tried a few things and found out that my issue was caused by Little Snitch (5.0.4). When I disable the Little Snitch network adapter in system preferences the sync work also in normal Mode. I wrote the Little Snitch support. I will report here when I got an answer…