Better late than never, I suppose. I had the same problem and I stumbled across this post. This is hoping others find it useful.
Check if you have to use a proxy server to access external URLs. In that case, add the following to web.config
<system.net>
<defaultProxy>
<proxy usessystemdefault="False" proxyaddress="http://your-proxy-name.domain.com:port-number-if-any" bypassonlocal="True" autoDetect="False" />
</defaultProxy>
</system.net>
I had a similar problem and this worked for me.