Hmm, Submitted bug #866795 too (contacts & CardDAV also broken), but suspect that's a duplicate of this, and that the real problem is with gnome-online-accounts, and is ultimately an unsolved upstream problem as described here:
"Add separate components for CalDAV and CardDAV accounts" https://bugzilla.gnome.org/show_bug.cgi?id=720519 .. if that's correct though, isn't that a pretty major regression? Dropping support for CalDAV/CardDAV is certainly something that should have been mentioned in the stretch release notes. C.
