I get the same problem with frustrating frequency. I pay extra for sky movies and every time I try to watch a movie on demand I get that error message. I’ve phoned, tweeted and emailed my complaints but no resolution months later. Have you got it sorted?
Does SD/HD make any difference? And which TV box are you using?
(I suspect I know the answer to both questions, but it would be far better for you to confirm)
Oh, and is it a permanent error?... Or does trying a bit later work? (I appreciate it doesn't help with "On Demand" but may well help with diagnostics)