Thanks for this report. However, I don’t know what to do with it. The error is a 500 Internal Server Error, and I don’t see anything wrong with the iCalendar data.
The expected behavior is to accept the iCalendar resource instead of sending 500 Internal Server Error. I could reproduce the problem for some files, but I don’t know why mailbox.org doesn’t accept these files. Maybe there are server log entries with further information.
Until there’s any hint that this problem could be related to DAVdroid, I can’t do anything for you. Please get back to mailbox.org / Open-Xchange.