Using org-caldav-sync, which connects to an https server, I was
consistently getting errors whose root cause was
url-retrieve-synchronously returning an empty buffer. *URL-DEBUG*
looked like:

  ...
  http -> url-http-end-of-document-sentinel in buffer ( *http XXX.example.org:443*)
  http -> Marking connection as free: XXX.example.org:443 #<process XXX.example.org>
  http -> Activating callback in buffer ( *http XXX.example.org:443*)
  retrieval -> Synchronous fetching done (#<buffer  *http XXX.example.org:443*>)

The buffer returned by url-retrieve-synchronously was empty.

It seems that url-http-end-of-document-sentinel does not set the
variable url-gateway-method to 'tls before calling url-http when using
https. Changing url-http-end-of-document-sentinel to set
url-gateway-method to 'tls if the url has scheme https fixes the
issue.

Patch for a proposed fix, which works for me, is attached.

I believe this is also the issue report here:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=9592