Errors when attempting to pay / register

HI Folks,
Sorry if this is a school boy error, but I'm struggling a bit here.

If a registered user (Myself) attempts to purchase a product there is an error page . In the page title it says 0 - Error: 0

If an unregistered user attempts to register they get a similar error at this address - www.foldyourownbook.com/index.php/hikash...out/task-step/step-1

Before I connected the site to it's domain name everything worked correctly, so I'm guessing it's an addressing issue but I don't know where to edit the actual URL