Welcome to our community cheeseboard,
I'm sorry to hear that you're getting an unknown error message. I will be happy to help check this out for you.
I have checked in the office here and it seems to work ok for us. Is this still an issue for you today? If so let me know and I can help further.
Also if you can let me know if this error comes up at any other point in them enue system this may help us figure this out 
One other thing you can try which may help resolve this, is to carry out a service connection. To do this follow these steps: HOME > HELP & SETTINGS > SETTINGS > NETWORK >CONNECT TO MY VIRGIN MEDIA SERVICE NOW.
This process may take a few minutes to complete, once done give On Demand another try and let me know if this has helped at all.
I look forward to hearing from you.
Collette