Step 2 of Checkout not working

I am using ZenCart 1.5.0 and in the last two weeks the checkout process has stopped working. All prices are shown correctly at checkout and then at Step 1 the shipping rate displays but when we get to step 2 all the items show $0.00.

When I revert to the default Classic Contemporary Green template it starts working again. The client assures me they haven't changed anything in the page files although that might not be true. I have tried reloading files in the custom template but without success. I have also checked the logs which say the following in most of them which seems related to tax, but again if it works in the default surely this file applies to both?

[07-Jun-2015 11:44:36 UTC] PHP Warning:  Invalid argument supplied for foreach() in /webaddress/public_html/includes/modules/order_total/ot_group_pricing.php on line 66

but I expect the error is coming from the custom template if it functions correctly in default mode?

I would appreciate any pointers on where to look - this particular website has been running well since 2011 and version 1.5.0 since 2013 and I have built numerous other ZenCart websites over the years but this one has me stumped.

Obviously the cart is a couple of versions behind, but I want this sorted before I can convince the client to upgrade to a responsive template and then upgrade the version as part of the deal.

The website link is http://www.afloralaffair.co.nz/ but currently it has been set to display mode only (due to cart not working correctly.

Thanks in expectation.

Were the "custom" template files created specificallly for Zen Cart 1.5.0 (or were they originally created for a previous version of Zen Cart)?

Have you checked for modifications to the website including both files and settings? Was the "group discount" order total module always enabled?

PHP version on the server?

Answers to the questions found in the posting tips (answers help the volunteers here hekp you)?

NOTE 1: Many hosting providers are no longer supporting PHP 5.3 (obsolete; EOL; no patches / fixes) and have upgraded their servers to only allow PHP 5.4 (obsolete; EOL in Sept; security patches only) or PHP 5.5 (currently supported). If your hosting provider has upgraded PHP, you will need to upgrade to a version of Zen Cart compatible with the installed version on PHP.

NOTE 2: Some older templates (and custom themes) may also have compatibility issues with modern versions of PHP.

NOTE 3: One can compare the "custom" templates against the "default" templates for the installed version of Zen Cart (download a clean copy first)... This may be a potential starting point for "fixing" the current "custom" template (as you have indicated the "default" templates are working)...

Also, because page files are not template dependent (no override) then the comparison is as lhungil suggested, template overrides to default files, also would want to look at differences in the directories for classic in the event there are files there that are not in the override and are the ones making the process a success when in default.

There are potentially other things to consider as lhungil stated, but more info would be needed to offer applicable suggestions.