Can't change appointment duration after creation

Reported here:
https://forum.kopano.io/topic/143/error-messages-when-creating-calendar-entries-in-outlook-2016

Steps to reproduce:
1. have OL synchronized against Z-Push
2. open the calendar view
3. create a new appointment via context menu on the calendar -> "new appointment"
4. after hitting save & close, the appointment is selected/highlighted in the calendar
5. drag the lower border to extend the appointment duration

Result: On release of mouse button after dragging, you get an error message that the message can not be saved because it was edited somewhere else.
This does not happen when disabling KOE and repeating the steps.

It didn't always occur, but most of the times in my tests.

Report was also about a message related to reminders I wasn't able to reproduce.
The forum post contains some other interesting findings related OLs overwrite protection.