So, found a solution (finally) for the annoying triangle in my OS X Calendar. It’s been there quite a while…
For anyone who has been working with Zimbra for a longer time (say, since version 4.1) you might have noticed that in it’s OS X counterparts Calendars and Reminders seem to integrate. This was obviously because of the CalDav protocol which resembled the webdav protocol too much. Opening a browser and going to your webdav URL also showed (reserved Names) next to your Calendar names, also the tasks names. However in the new OS X Reminders app, instead of showing the tasks name from webdav, it ALSO showed the Calendar names.
This means: Zimbra Calendar had a dual function (unintended): appointments and reminders. Of course, in later combinations of zimbra and OS X this was fixed. Some while ago – when iCloud was introduced – this annoying warning in my calendar list appeared, which stated “CalDAVAccountRefreshQueueableOperation” failed. Yet the calendars seem to work fine, as does the Reminders list.
I finally put 1 and 1 together with thanks to this article from 2014: https://discussions.apple.com/thread/6602331?start=0&tstart=0 the conclusion was: Zimbra & iCal have individual enable/disable checkmarks for tasks/reminders synchronisation to the cloud. But apparently there is a third one. I was looking in Internet Accounts, and should have been looking at the iCloud settings. And even there: in the iCloud settings from 10.12.5, reminders was unchecked?
But that setting was not the issue, no, you need to go INTO the iCloud Drive options, and there disable Reminders.
That did the trick; closing and opening iCal – everything still works but no more annoying error….