Strange situation. I can watch Slingbox just fine when on the same network where the Slingbox is located. It still works even if I use a different network but am in the same building as the Sling.
As soon as I leave these two networks and try to login from a different location, I can hear the Sling audio but get a black screen with no error message. This is the same result with a variety of different devices and remote networks.
I only have a coax cable going to the Slingbox. I've rebooted the box and the router.
I've checked the router and it is set to UP&P as suggested. What else can I do?