As I explain in this thread, my PR1.3 N9's CalDAV client might not be in compliance with the CalDAV specification. It could not delete calendar events stored on my DAViCal CalDAV server. When it attepts to delete a calendar entry, it sends a DELETE message with an empty message body. DAViCal rejects this as an ill-formed message. To be compliant, the CalDAV client must send the DELETE message without any body at all. A simple DAViCal patch fixes the issue for my server. I don't know if other servers behave this way.