I am having a similar problem... Well, it let me register my nabaztag/tag just fine, but I think my serial number is already in use (I bought her new, but the box was opened. Very strange).
For curiosities sake, have you tried registering in a different browser? I notice Violet renders things very differently in different browsers, so it might help?
In the meantime, I am also waiting on Violet tech support.
Hope that they can help us both.