Checkout_Shipping will not advance to Checkout_Payment

Using ver 1.3.8a
Linux Server, PHP 5

Store has been working fine, got a report today checkout does not work.

When I tested, the Checkout_Shipping page will not advance to the Checkout_Payment page. When clicked, I get a blank screen and it appears to try to load Checkout_Shipping again, though it stays blank.

Same in IE8, Firefox, Safari.

If I manually update the url to page=checkout_payment then the order proceeds and I can complete.

Using FireBug I found the html for that area...the form submit section lists the post action as https://www.teltechservices.ca/store/index.php?main_page=checkout_shipping

If I use FireBug to change this to https://www.teltechservices.ca/store/index.php?main_page=checkout_payment the order proceeds and I can complete.

So I think I have identified the problem...but after hours of searching my installation and the forum I can't figure out which file is writing the html for that page so I can fix it.

the site is https://www.teltechservices.ca/store if you want to look at it...just create an account to try it out...you don't have to complete the order to see the problem.

Thanks

https://www.zen-cart.com/tutorials/index.php?article=82
https://www.zen-cart.com/tutorials/index.php?article=87

Thanks DrByte

Using the debug process led me to the file and the line number of the problem...which was the last line of the file.

Looking back at the page you sent me to it says that if the error is on the last line then the problem is an extra space, sure enough, pressing the down arrow revealed 2 extra spaces at the end of the file.

Ironically, if I had searched for 'blank page' instead of focussing on the checkout_shipping I would have found the answer much sooner....sometimes being too specific is not a good thing.

Anyways, many thanks for the help.