I have no idea if this will help you but since I came here looking for help, didn't find any and stumbled on to something I thought I would share.
I have an X1 and hooked the box into my TV with an HDMI cable. I hooked the slingbox up through the five RCA setup. I noticed the box worked fine when the TV was on. When it went off, I got the UNABLE TO PLAY VIDEO message.
So I figured I would pull the HDMI plug from the X1 box and see what happens. Well, the second I did that, the UNABLE message went away and the Slingbox worked.
I just did it, but it streamed fine for 10 minutes. Give that a shot.