I had a very old install, migrated from 2016. It broke on nearly every upgrade, where I'd have to invoke some occ command to fix the db. It was more work than was worth having Nextcloud.

Now I have syncthing and radicale.

Not the pne you replied to, but for me it works to add the full path to another users calendar in davx5, and then setting appropriate permissions to it in the rights file.