RFC 6638 requires CalDAV servers to allow principal property search based on a principals calendar-user-address-set:
https://tools.ietf.org/html/rfc6638#section-2.4.1
GitMate.io thinks possibly related issues are https://github.com/nextcloud/server/issues/11540 (User search), https://github.com/nextcloud/server/issues/11256 (personal calendar for newly created users has no calendar color set), https://github.com/nextcloud/server/pull/11886 (Principal property search for users and groups with {DAV:}display name), https://github.com/nextcloud/server/issues/5903 (CardDAV: principal-property-search delivers no results), and https://github.com/nextcloud/server/issues/4471 (Set share_folder per user).
@georgehrke Let's move this to 16, okay?
Yes, let's wait till 15 is out. In case of doubt we can still back port it if necessary.
Moved to 17