Checkout Error

where do I start to narrow down to which of those issues it could be?

This is the error URL that appears in the address bar on the white screen:
'https://www.bow-shop.com/secure/store/index.php?main_page=checkout_payment&error_message=%20-%20Your+card+has+been+declined.++Please+re-enter+in+your+card+information%2C+try+another+card%2C+or+contact+the+store+owner+for+assistance.&zenid='

Not entirely sure. Might be using a module that's not built for v1.5.0 or even 1.3.9.
If the module is one that sends customers offsite for payment, then you probably need to talk to the gateway company.
If the module collects payment details directly on your site, then you probably need to rewrite the business logic in the before_process() method of the payment module class, ie the code in the includes/modules/payment/psigate.php (or whatever the file's name is)

Yup, that's definitely an outdated payment module. Designed for pre-1.3.8.

You'll need to rewrite the logic that puts the error into the &error_message parameter to put it into the $messageStack instead.
Long error messages like that will automatically trigger a blank page because long parameters are commonly abused by attackers.

I know the module is built for 1.3.9 because I used it in my past shopping cart. Zen-Cart used to be listed as an eCommerce partner with PsiGate but I see it no longer is. So could very well be that it is acting a little different with the new v1.5.0

PsiGate was an included payment module with 1.3.9h, which I used before and why I signed up with them to begin with.

But I have a place to start and see what I can learn, which is what I am looking for

thank you

DrByte:

Yup, that's definitely an outdated payment module. Designed for pre-1.3.8.

You'll need to rewrite the logic that puts the error into the &error_message parameter to put it into the $messageStack instead.
Long error messages like that will automatically trigger a blank page because long parameters are commonly abused by attackers.

I see you have responded to something like this in the past, can I assume that this is the same situation as the following thread?

http://www.zen-cart.com/showthread.php?183478-Credit-Card-Error-Messages-How-To-Create

Yes, that's the concept.