I used CalDAV which was working well.
After changing the URL as you mentioned above, I am still not able to connect.
However, I use my credentials from CalDav, which worked well on OH2.5 and Caldav.
Did you ever make it work with user / password?
EDIT:
When using curl with the same adress according to this thread I get all my calendar data:
Anyone who has running this on OH3 with user / passwd?