I've just installed a Zen Cart with the FedEx modules set and the Flat-rate mod turned on. When I try to checkout no matter what shipping method I select I get the following error when I click the continue button to go to the Payment part.

Warning: Cannot modify header information - headers already sent by (output started at /usr/home/zoo42/public_html/wscart/includes/languages/english/modules/shipping/flat.php:53) in /usr/home/zoo42/public_html/wscart/includes/functions/functions_general.php on line 44

I'm using version 1.3.7.1 of Zen Cart.
I have another cart using version 1.3.7 with the same modules set and everything works fine.

Can someone please help.