Hi James.
My first instinct is to ask have you made sure you have enabled the Decipher Web Site? By default after you install it, it is not enabled. Once you have installed the Decipher Web SDK you have to go into the IIS manager and start the Decipher website.
If it's not that then I would suggest raising a support ticket, or maybe better an Expert Connect session, as there are a lot of variables here that might be contributing.
Thanks