CalDAV sync Outlook->NC->iOS: some outlook event synced to NC, but won't be displayed on iOS

System information
Nextcloud version: 16.0.3:
Operating system and version: Unknown (Hosted webspace)
Apache or nginx version: unknown
PHP version: unknown (7.x)
Outlook: 2016 (16.0.9126.2388) 32-bit
CalDav Synchronizer: 3.6.2
iOS: 12.4

The issue you are facing:
I’m using CalDav Synchronizer in combination with Nextcloud to sync my Outlook events to my iPhone.
For 99% of my events it works fine.
But with some events I have a problem: This events will be synced with between Outlook and Nextcloud, but the further sync to iOS is broken for this few events.

I’m not sure why especially this events have a probleme.
Does somebody knows how to isolate the problem? Are there some log files?
Or maybe is this problem already known?

Thx

If you can see one of these problematic synced events in the Nextcloud GUI, you should export it to an ics file and post its content here. As a next step you should create an event on your iOS device, sync it with Nextcloud and export it to an ics file too. As next you can compare both files to find out how they differ.