Check_Process Errors

So I just upgraded to v1.5.4 in order to fix the PayPal Express Checkout problems and now my checkout-process is not working again. To fix this issue in the pass I had to rollback zen cart to v1.5.1 while keep the 1.5.3 database, but now that will not work as the PayPal issues will return. Someone please help! Is there something which has changed between ZC checkout_process between the upgrades between v1.5.1 and v1.5.3 and beyond and how do I fix it? Also I have noticed that there are no order confirmation emails being received following the test orders although they show up in the ZC admin.:frusty:

When logged to do a test order I get the following once the order is confirmed:

When that error message appeared, you were completing payment for an order. Can you please explain exactly which payment method had been selected for payment of that order? And which specific payment module handles payments for that method?

... because that error message does not come from any of the built-in modules that come with Zen Cart, including PayPal.

It appears that what you posted today comes also from the same payment module you were having problems with from before the upgrade, so I've merged today's post with the previous discussion, above.

DrByte:

When that error message appeared, you were completing payment for an order. Can you please explain exactly which payment method had been selected for payment of that order? And which specific payment module handles payments for that method?

... because that error message does not come from any of the built-in modules that come with Zen Cart, including PayPal.

Usually when I do test payments I choose payment by check/money order. I have also completed a payment using Paypal and it gave the error upon returning to the site or it sends the customer to a blank page.