Changes in the dav URLS in nextcloud break integration into gnome (e.g. no calendars or contacts showing up in evolution): https://docs.nextcloud.com/server/11/admin_manual/issues/general_troubleshooting.html#troubleshooting-webdav
See this link for further information:
Further comments: contacts/calendar integration stops working on existing accounts and on new accounts (no error message on adding a new account, webdav file access still works)
Deleting, re-adding the account, AND waiting for a significant time, solved the issue for me. Sorry for the noise.