I just encountered this situation. It affected my calendars and my address book. Once the initial panic subsided, I made similar determinations as those shared in the OP. DavX5 (FKA Davdroid) was working. Only Thunderbird was failing.
I had just added Duo and OAuth to my ownCloud the night before, so I spent a little more time than necessary focused on app passwords. Once I found the 401 response in the Thunderbird error console, I performed further diagnosis using private browsing of the DAV URLs in Firefox.
I was able to implement a workaround from the following findings. The URLs as shown in ownCloud are listed below under the label of ‘not working’ since they failed to trigger an authorization request and just failed immediately, The ones labeled ‘working’ provided the necessary HTTP authorization prompt.
CalDAV
Not working:
https://owncloud.example.com/remote.php/dav/calendars/[username]/defaultcalendar/
Working:
https://owncloud.example.com/remote.php/caldav/calendars/[username]/defaultcalendar/
CardDAV
Not working:
https://owncloud.example.com/remote.php/dav/addressbooks/users/[username]/contacts/
Working:
https://owncloud.example.com/remote.php/carddav/addressbooks/[username]/contacts/