@peters Thanks for your explanation. I haven't heard that Synology uses mod_caldav yet (to be honest, I didn't even know that such a module exists). This might be useful information; maybe we could file a bug for mod_caldav. Maybe you can have a look whether such a bug report already exists?
However, it's quite clear that the response for the REPORT should be an empty 207 and not 404, so I don't see the need to change anything in DAVdroid.