Sky Box Office you pay for individual films. Sky Movies is something completely different and you should get that if you have the Sky Movies package with your box. This will only work if you have a TiVo however.
Sounds to me like a problem with your connection if you are getting this message all the time though. Hopefully either 1 of the moderators will come along to help you as you don't seem to be getting much help phoning for help. Maybe worth selecting the thinking of leaving option and then you should get someone in the UK who I have found very helpful every time.
Welcome to the Community, I'm sorry to learn about the problems you're experiencing with your On Demand service lately.
I've been able to locate your account and run some tests on the set top box from our end for you, we're not detecting any issues at the moment although I can see you've very recently rebooted the box, previous to that I can see a few errors but since the reboot, things are looking well.