Sorry for the late reply but I have just had the identical problem and managed to solve it. Maybe it will help other people who have the same problem.
The remotes themselves are stored in C:\Users\.....\AppData\Roaming\Slingmedia\WebSlingPlayer\Remotes If some reason the files get deleted or maybe a new user is added, then the Slingplayer should automatically reload them when the associated stream is viewed. But it seems that for some of the remotes (not all) that is where they 'hang' at 50%. The solution is to run the "Reconfigure Input" routine. Then the remote is then correctly downloaded.
Moderators: Please pass this onto the engineers. The remote that displayed these symptoms was the Sky+HD box.