Why don’t you just select no address book for sync? There’s no obligation to select an address book in the Account view.
DAVdroid fail to connect to URL containing "~"
Try connect to https://example.com/~user/owncloud/remote.php/caldav/
“HTTP error: 403 Forbidden”
No problems to connect using desktop browser.
Duplicate of #80.
Please see also Why tilde (~) should not be used in Web addresses (URLs).