There seems to be a bug with the new Sophos 10 AV software that prevents you from connecting to your sling box.
When trying to connect to your slingbox via the slingbox website, I was getting a "There was an error connecting to the box. Please check your network, firewall and router settings. Do you want to retry". Repeated retries does nothing. Only until I disabled the Sophos Web Intelligence service did it start to work again. This problem only seems to effect the new version. Version 9.7 works fine.
I contacted Sophos and the are aware of the problem. Hopefully they will fix it soon.