You obviously haven't been watching the forum, there has been a lot of discussion about this. Sling have redesigned their systems so that the Desktop player will only stream the M1/M2 and reconfigure all other models. If you want to watch a stream on any Slingbox except the M1/M2 you need to use the browser player.
I have been watching a Slingbox Pro without issues for many years. Recently, I added an M2 to my network. There is no problem watching the M2 on the web, but Pro no longer works with the desktop app. It asks me to reconfigure the Pro. (It apparently can't be watched over the web.) After a hard reset and new setup, it still asks to reconfigure. There seems to be no way to watch the Pro anymore. Does anyone have any suggestions?