Integrate CalDAV-Server-Account (Baikal) in Apple Calendar over TLS with Client Certificate

When I try to create an new CalDAV-Account to access an calendar on a Baikal-Server, the process does not present the requested client certificate to the Web-Server.

All certificates are valid and trusted. Applications have access to the certificate in Apple Keychain.

But the logs show follwing error:

2025-12-27 16:47:11.990201+0100 0x2eef6    Default     0xc4837          
    3754   3    InternetAccountsSettingsExtension: (Accounts) 
[com.apple.accounts:core] "Completed account credential verification: 
(null) - Error Domain=DAAccountValidationDomain Code=100 
UserInfo={NSUnderlyingError=0x7091a9fe0 {Error 
Domain=CoreDAVHTTPStatusErrorDomain Code=400 
UserInfo={CoreDAVHTTPHeaders=<private>}}}."

Any hints?

The only post I could find is this 15 years old:

https://discussions.apple.com/thread/2654950?sortBy=rank