Thanks for the idea about how to clear the sling adapter problem fastford19 by: Choosing what you want to view on the remote device or web site, going into the Hopper's Settings -> Diagnostics -> System Status, and after a few seconds exiting out (don't stay in too long, or the client will report that a user is performing a switch reset).
Here's what I think is happening:
When you first connect to the Hopper via Sling, the Sling adapter is RANDOMLY connected to an available tuner on a RANDOM channel, and then it switches to the desired thing (the desired channel, or a DVR recording). If that initial randomly tuned channel happens to be one that you DO NOT subscribe to, game over, the Sling will never get past an unseen "you are not subscribed" situation and you're dead. (You can verify this stuck condition by pressing the RED button on the remote to show all the tuners -- you'll see SLING_SSA tuned to a channel you're not subscribed to.)
Unfortunately, even rebooting the receiver does not often cause the Sling to choose a different (hopefully subscribed) channel. I think the reason that fastford19's solution above works is because viewing the system status disrupts/disconnects all of the tuners, including the Sling's, and then the app (or web site) can now have a better shot of getting the Hopper to tune the Sling adapter to a different (hopefully subscribed) channel and not get stuck.
Of course the solution only works when you're at home, so its use is limited to using a computer/phone/tablet at home -- if you're on the road, you're stuck.
PS. Dish Support says they'll be pushing an update out to correct this next week.