Continue Checkout button not working with Masked URL

I have a domain forwarded to my website with masking,
the client purchased the domain, and is using our hosting, so we used the domain forwarding from network solutions to our server.
when i go through the check out process,
i add product to cart,
click check out
on the following page click check out now
then on the shipping page, enter information and click "continue checkout" and then there is nothing but a blank page instead of forwarding information to paypal to continue the check out process.
is there any way to remedy this, or a reason this would happen ?
example:
http://hookdatstuff.com/store/products/purple-n-cold/

but when i go direct to the products and through the check out with a direct link to the account that's actually hosting the site, i have no problems checking out and forwarding to paypal and back.
example:
http://www.thegraphixlab.com/hook2/current/content/store/products/purple-n-cold/