I've a webapp that serve an ical file made with ical4j
I use CalDAV-Sync on Android to access the calendar through HTTP (Webcal)
CalDAV-Sync seem to make HEAD call first which should return something but I don't know what...
Then CalDAV-Sync doesn't do the GET to get the actual calendar....
Do you have an idea what should the HEAD return?
CalDAV-Sync is outside my field of knowledge right now, but you may be able to find some information here:
Otherwise maybe post your question on stackoverflow.com or somewhere like that.