I am having the exact same issue as described here:
Having a proxy server configured in XenCenter connection settings and trying to check for XenServer updates one ends up with an empty list. The reason is quickly found in the logs
of the squid webproxy:
TCP_DENIED/403 1882 CONNECT updates.xensource.com:80 - NONE/- text/html
XenCenter tries a CONNECT request on port 80 (http) and not on port 443 (https), which gets denied by the proxy server (tunneling is only allowed for https traffic, which is very common procedure). When trying to access http://updates.xensource.com IMHO a simple GET request would do the trick.