Sorry, I've never encountered that error when using clean v1.3.8 code.
It's not clear to me what you did to get to the point of having "zen-carts latest version onto my site now.. "
How does your site differ from a clean install of v1.3.8 ?
Also, I'm not certain *which* payment module you're actually using when you do the steps you're describing. You don't need the Website Payments Standard module enabled if you're using Express Checkout.



