I am having similar problems. Everything was working fine a few weeks ago. I could watch from slingbox.com, but now the "newwatch.slingbox.com" returns message that my slingbox needs to be configured. I download the desktop app, configure the Slingbox Pro HD (again) ... everything was working, nothing needed to be changed, and then when I go back to "watch" on slingbox.com, it forces me to the screen that my slingbox needs to be configured.
Same behavior on Firefox, Chrome, and Edge, all under Windows 10 Pro.
Sling was purchased by Dish and they aren't interested in supporting older SlingBoxes.
I downloaded this from a different forum
This is an old version that works with my Slingbox Solo - hopefully it works for you as well.
Here is the old slingplayer download link
There may be other versions available, but there doesn't seem to be an index to the other downloads. We need some help from other users who have found other versions to download since Dish doesn't want to help.