v139h PxPay Issues

Hi guys

I'm trying to setup the PxPay module located here http://www.zen-cart.com/downloads.php?do=file&id=658 on our site, I've installed it and set it up with our PaymentExpress account, however when I try checkout using the module I'm redirected to the PaymentExpress page correctly, enter the test CC number and have the transaction approved. But this is where it goes wrong, on clicking next I'm not redirected to the order confirmed page on our zen-cart but just a blank page as if there is some sort of PHP error or something. (but no errors appear in the cache folder) (also the order is not complete and the item is still in my cart)

Next I setup the site on a local WAMP test server to try troubleshoot the problem, but here it works perfectly!! redirected to the order confirmation page and the order is created!! thus I believe the issue is with the server config.
My local server is running PHP Version 5.3.13 and the online server is running PHP Version 5.3.8 and both have CURL installed correctly (I think). the php versions are pretty similar so I don't think that's the problem?

could anyone give me any tips on how to track down this problem??

Thanks!
Rob.

EDIT: basically the same issue as this: http://www.zen-cart.com/showthread.php?194116 I'm thinking it has to do with the url return length...