macOS Server calendar delegation not finding users We're running a Mac Mini in a small business network, which hosts (among other things) a calendar server using Server.app. We have set up accounts for each user, so that they can sync calendars with the server.

In Calendar (on a user's Mac), one can supposedly share calendars with other users on the same calendar server. This is done by right-clicking a calendar, choosing 'Share calendar' and typing a user's account name. Alternatively, the calendar can be delegated via the 'Accounts' preferences (Caldav > Delegate > Edit..). However, neither of these actually find any of my server users! The dropdown only shows names from e.g. Mail.

I've tested with both 'local directory' and 'network directory' users (i.e. with and without Open Directory), but neither show up. Preferably this would work without using Open Directory (as I have no other use for it, and simple is better than complex), but I'd set it up if required. 

What does this delegation depend on? How does it do user discovery?

EDIT: When attempting to do user discovery, I do see the following log entry:

"REPORT({http://calendarserver.org/ns/}calendarserver-principal-search) /principals/ HTTP/1.1" 207 1229 "-" "Mac+OS+X/10.11.6 (15G1217) CalendarAgent/361.2" i=3 or=1 t=80.2 fwd=&lt;client IP here&gt; unix=true


And browsing to /principals (i.e. HTTPS on 443) shows a bunch of stuff, but gives me a permission error when clicking /principals/users.

EDIT2:

When browsing to https://mydomain/webcal, I can actually add delegates using the web interface! However, it appears that these delegates are 'per user' rather than 'per calendar', and the delegated agendas show up differently in Calendar.app. They do not appear to sync to iOS Calendar at all, either. Not ideal, but closer.
This problem started at a client of mine after they upgraded clients from 10.6 to 10.7 and up. Calendar was stable before this. Webcal finds all ldap users, calendar app on clients does not find everyone, and I have yet to find a pattern when who doesn't find someone else. Some cannot find very old ldap users, others cannot find an newly created ldap user. This when creating calendar events and inviting other users. By the looks of it, a bug in the calendar.app in macOS.