Dozens and dozens of people I would hazard a guess you didn't do a Forum Search before posting ? We have been drowning under posts about this. Your TV Provider has implemented HDCP (Google it). The solution is to throw away the HDMI cable and feed your TV from the Component OUT plugs on the Slingbox.
I have had a sling box 350 for 2 years now on the same television. It has always worked without the television needing to be on, but now I see an alert from my cable company saying that the hdmi connection (from my cable box to television) is not working. I am able to use the remote, and I can see the channels changing, but the alert remains. Then when the television is turned on everything works fine. Once it is turned back off, I receive the hdmi alert error. Has anyone had any luck with a similar issue?