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.