Thad, does the Classic stream programs properly? If it does, then there could be some problem with the way it interacts with the Setup Assistant. If it does not, then that could indicate a broader problem.
Also, if you see this error using Setup on Slingbox.com (the web Setup Assistant), do you see it using the desktop version of SlingPlayer, either for Mac or for Windows? Or if you are using the Desktop version, do you see it using Setup on Slingbox.com? (http://setup.slingbox.com)
I would also try to find out if there is a firmware (Sling Media calls it "Slingbox Software") update for your Classic. If so, then the absolute first thing to do would be to update that, because if it's outdated, then there's lots that won't work right.
It is true that Classics are getting long in the tooth ... but they still do work for a number of situations. But they are getting old. :-) Good luck.