Overview

You are unable to check availability of other users in the Apple Calendar in macOS BigSur or later, via the CalDAV protocol. All users are shown as available, even if they are marked busy.

Solution

This issue could be arising due to Apple modifying its CalDav implementation in the Big Sur release.

Please note that an enhancement request to add support for the Show Availability functionality over CalDav for Big Sur and later has already been raised to the Development Team for consideration in a future release. 

You can track the same through the Kerio Connect Release Notes page. Please do note that an ETA is not available for this request.

In the meantime, you can set up Apple Calendar using an Exchange connection instead - the Check availability functionality works over EWS in the Big Sur and later release. For guidelines, please refer to Setting Up Exchange Accounts on macOS. 

Note: In Step 7, you should only select Calendars, if you only wish to have Calendars set up over EWS.

Alternatively, you may use a 3rd-party Calendar application like BusyCal for Mac - the Check Availability functionality has been tested to work on it over the CalDav protocol.