Hi Igor11 and thank you for your comment. Yes, Apple should definitely look into this issue. I tried contacting them on developer and customer support and feedback channels, but they have not yet replied...

hi bydict, are you using sabre/dav as caldav server? I tried what you said, but my url was already in the https://abc.com/ form, no relative path needed... I tried some different combinations, based on your answer, but nothing seems to work... We need help from apple, but it seems impossible to get in touch with them

yeah I already looked for something in the release notes, but there's nothing about caldav