I emailed this to support, and they referred it to their 3rd line support. I guess they will fix it - or not. There is not much I can do now but wait and see if it starts working at some point. I guess this is not a common problem, otherwise some user-agent spoofing option could fix this more generally(?)

@rfc2822 I’m sorry, I can’t. But my fantatstic admin was able to! All is back to normal now, thanks for assistance.

Hello, 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.