Changes in recurring appointments are not visible on other devices

[Christian Wolf]

Dear mailinglist,

I have the following setup:
There is a nextcloud server with the current calendar extension. I configured 
KDEPIM (multiple installations) to synchronize with these calendars. Further I 
have a set of other devices (foremost Android mobile and tablet) that sync to 
this calendar as well.

For normal appointments this setup works well so far.
In case of recurring appointments there is a problem with exceptions.

I have 3 categories of devices that share the calendar:
- KDEPIM installations
- Nextcloud live preview of the calendar (web frontend)
- Android devices

Lets assume I have a weekly appointment to go to sport every Friday evening. 
Due to Corona, I cannot go for a few weeks. Thus, I remove the appointments.

If I remove the appointment from my mobile phone, it gets synchronized to the 
web frontend (the appointment vanishes at the removed instances). In KDEPIM 
this is NOT reflected. The appointment is still fully visible.

If on the other hand I remove the appointment in KDEPIM, it is neither removed 
in nextcloud web frontend nor in the mobile device.

I tried to investigate a bit. There seems to be different ways to store 
exceptions in recurring events in CalDAV (see https://www.kanzaki.com/docs/
ical/exdate.html). I looked into the raw payload of the event in 
akonadiconsole. Unfortunately, the value was changed during synchronization 
very quickly. I could only get a short glimpse that something has happened but 
I was not able to read anything useful. I just saw that the string defining the 
exception was not formatted the same as the others.

Another try revield the following:
KDEPIM seems to generate exceptions like this:
> EXDATE;VALUE=DATE:20200325
The other two solutions use another syntax:
> EXDATE;TZID=Europe/Berlin:20200325T220000
> EXDATE;TZID=UTC:20200326T210000

Does anyone have a similar setup? Do you have the issue as well or not?

I am a bit lost as it is not fortunate that things are not working smoothly 
but I do not know which component (android/nextcloud/kdepim) is causing the 
issues.

Thanks in advance
Christian