Accessing google calendar from an iphone using caldav

I would like to access to a google calendar via a CalDAV account. I have been doing this successfully for a number of years, but this week it suddenly stopped working. I assumed at first that either (a) somehow my app password expired, or (b) there was some sort of organization policy change preventing caldav access, but experimenting with curl shows that this works just fine:

curl -X PROPFIND -u "..." \
  https://www.google.com/calendar/dav/me%40example.com/user 

This returns a successful caldav response:

/calendar/dav/me%40example.com/user
  
   HTTP/1.1 200 OK
   
    Lars KS

And running:

curl -X PROPFIND -u "..." -H Depth:1 \
  https://www.google.com/calendar/dav/me%40example.com/events/ 

Seems to successfully access calendar events; it returns over 45000 lines of responses that look like

/calendar/dav/me%40example.com/events/11223344-6819-427c-9ee0-ccd8642b1a96.ics
 
  HTTP/1.1 200 OK
  
   text/calendar; component=vevent
   "63566811207"

Great! But if I go into the accounts settings on my iphone and add a new caldav account, so that it has: | | | |---|---| |Server| www.google.com| |User Name| me@example.com| |Password| secret_app_password | |Advanced Settings -> Port| 443| |Advanced Settings -> Account URL| https://www.google.com/calendar/dav/me@example.com/events/ | Whenever I click "Done", it spins for a bit then reports:

CalDAV account verification failed

What could be failing here? The URL and credentials match what I'm successfully using with curl.