Hi Henryk,
Greetings from Syncfusion Support.
We have validated your reported query "exception throws on editing the following events" and confirmed this as defect at our end. So, we have logged the bug report which can tracked from the following link.
The issue fix for this defect will be included in our volume 3, 2021 release which is expected to be rolled out by end of September 2021. We would appreciate your valuable patience until then.
Regards,
Nevitha