Thanks for your report. I agree and this is not intended behaviour as certificate errors are hard errors which should be shown. Please follow up in #507.
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).