I was able to trace down the error by increasing log_level to 6. In fact, there seems to be one calendar item causing the issue:
The content of that ics file, which can be made available by simply downloading it with a browser, is just:
This seems to be enough to cause the iCal server of Kopano to stop with the errors:
Thu Aug 10 22:44:27 2017: [error ]  Error converting mapi message to ical, error code: 0x8004011b corrupt data
Thu Aug 10 22:44:27 2017: [debug ]  Unable to process report multi-get: 0x8004011B
Thu Aug 10 22:44:27 2017: [error ]  Error processing REPORT request, error code 0x8004011b corrupt data
Thu Aug 10 22:44:27 2017: [debug ]  > HTTP/1.1 500 Internal Server Error
As I cannot upload calendar content here or at the regular Kopano forum, is there a way to simply remove that single entry from Kopano by its ID? It has no subject or other way to identify.