I'm having the same problem. For some reason, the Slingplayer app doesn't support Chromecast when both my controlling device (iPad) and Chromecast are connected on the same iPhone hotspot wifi network. Youtube works great. It appears to be a bug in the Slingplayer app, because after initiating the Chromecast it quits with an error that reads: "Session has unexpectedly closed. Please check the Internet connection of your ChromeCast and Slignbox and try connecting again."
Slingbox team, please add support for hotspot wifi connectivity!