iOS 18.4 / iPadOS 18.4 breaks CardDAV / CalDAV - MacOS 15.4 works, iOS 18.3 and iPadOS 18.3 also works

Possible solution that worked for me:

open Settings > Calendar > Calendar Accounts > <CalDav-Account> > Account Settings > Username. Replace the @ in my username with a new @ and tap Done.

If you previously deleted your account (for testing) then recreate it using user/password, after getting the error message try to replace the "@" sign in the username and check again.

Same here! Neither CalDav nor CardDav are working on iPhone 16 Pro and iPad Pro M4. On MacOS it works on all my machines.

My company has a handful of users that still use our old Darwin based calendar (slowly getting them to stop, I promise) and we had this problem today. I think Apple may have stopped the calendar autodiscovery for this method as I didn't see the attempt in my access logs when adding my old account to my 18.4 phone.

To fix it, instead of just using the server.domain.com for the server, I had to do the full calendar path:

server.domain.com/calendars/__uids__/paulc_domain.com/calendar/

This worked right away and I was able to sync again. It worked after a phone power off and power on too.

Worked for me on three different devices. But it is possible that it depends on the backend if this "workaround" works. My backend is a mailcow server.

Thank you so much! this worked for me!