Certificate problem with MacOS Ventura 13.2.1

Hi all,
I just installed the actual NC client on my new Mac mini. Nextcloud is self-hosted via docker on an unRaid server. So there is a swag docker doing all that Let’s encrypt certificate stuff. Works like a charm for both my Windows clients and iOS devices. But I allways get a certificat error on macOS.

Screenshot 02.03.2023 um 14.40.45 PM

Screenshot is German but I think one can see what happens here. I’m using a CNAME entry for subdomain nxtcld.xxx.de pointing to DuckDNS. And here the problem starts I think as the client is expecting the DuckDNS domain to be covered by the certificate. I tried that be adding it to the additional domains in Swag with no success.
But why is that only with macOS client? Using Safari or Chrome opening nxtcld.xxx.de works a expected.
Any idea someone?