I have been struggling with this same issue ever since I upgraded my server with Yosemite and the latest server.app. I had many clients accessing one wiki calendar with no problems and now it won't work.
I had this kind of uri set in my clients CalDAV accounts: "https://server-address:8443/principals/__uids__/wiki-wikigroupname/" (which seems to be an alternative to the one you had) and everything just worked perfectly. But now it does not get anything.
So this made me wonder that how can this web calendar work, as it’s accessing to the same WebDAV resources. And while looking Apache logs it seems that web calendar is successfully responding (HTTP 207) to PROPFIND and REPORT requests on /calendars/__uids__/wiki-wikigroupname/calendar/ while same resource responds (HTTP 503) to different user agent (same credentials)
This really irritates me, because not only it’s not working with OSX Calendar, but also as it’s not working with IOS (8.1) devices either. And if you try to use the wiki web calendar with IOS device, you get a notification “Web calendars are not available on Mobile Safari. You can configure access from Calendar application on iPhone, iPod Touch or a Mac or view them in a Web browser on a Mac or Windows-based PC”.