iTunes unknown error -11821.

I've uploaded .mov unto iTunes, and when I click to create an iPad version I get an unknown error -11821. Does anyone know what the problem is?

I worked successfully earlier today, but took quite a long time. Thinking that it would help speed along the process, I purchased an downloaded the new operation system. No it's not working at all. I keep receiving the same unknown error code. My boyfriend gets the same error code when he tries on his PC.

Help!