Mac OS sync doesn't work for me

That doesn’t correlate to the information I have so far.
Can you post the link to the website where this message appears (anonymised)?

The URL of the website showing this message is “https://my.main.server.de/subfolder/remote.php/dav
And all Android and windows clients can use caldav/carddav - just mac os doesn’t get it…

Just to be sure:
when you visit https://my.main.server/.well-known/caldav you get redirected to /subfolder/remote.php/dav and you get prompted to enter your user credentials before the message “This is the webdav interface…” is shown?

If yes: how did you try to setup your iOS device?

It’s exactly as described by you and I tried to create a new caldav account and enter https://my.main.server.de/, Account-name and password …

Have you tried to delete the caldav accounts before adding it?

there is just one icloud account and every time I tried to add the nextcloud-account I deleted the existing nextcloud-account and tried to add it completely new…

Sorry, I am running out of ideas. Would it be possible to send the url via PN? And also a test account so that I can do some tests myself?

I have created a new account (without spaces) and now it works …

Good to know!