Can't use REPORT to check for updates

[ghost]

As mentioned on http://sabre.io/dav/building-a-caldav-client/ section "Finding out if anything changed" I have to made a REPORT call to /calendars/johndoe/home/.

But I get an HTTP 415 response code and the follwing message: 3.1.3 Sabre\DAV\Exception\ReportNotSupported

So how have I to check for changes (new events, delete events, edited events).