adding to Mathieu's answer I'd like to let you know that this seems to be a specific issue that we are hunting at the moment. Apart from you, one other user has experienced the problem that our viewer doesn't even initialize on his computer, independent of the browser. We will look into this early next week and hope to update our viewer once we know what causes this. We'll keep you posted.
When it works at all, it seems that the viewer works pretty consistently by now, at least that's our experience.