Hello All,
Hope this helps.
I was having the same problem - Sky Go stopped working and I got the 9903 message, even if it was the only thing I seemed to be running. So here's what I did.
I went to my applications folder and switched on QuickTime and then Quit it.
I went to the Activity Monitor and searched for QuickTime and it was there, so I clicked the little cross in the top right to stop the 'process' QuickTime vanished from the list.
Opened Sky Go App and it worked!
I think that some people think that because it's not open in the dock that it's not running (that's what I thought). But find it in Activity Monitor and shut it down and see if it works for you.