Thank you DrByte

There are only four differences between the 1.5.5 and 1.5.6 includes/filenames.php at the following lines

29
define('FILENAME_AJAX', 'ajax');
41
define('FILENAME_CATEGORY_PRODUCT_LISTING', 'category_product_listing');
56
define('FILENAME_COUPON_ADMIN_EXPORT', 'coupon_admin_export');
162
define('FILENAME_STATS_SALES_REPORT_GRAPHS', 'stats_sales_report_graphs');

The ezpages are these two, and are present in both versions
82
define('FILENAME_EZPAGES', 'page');
83
define('FILENAME_EZPAGES_ADMIN', 'ezpages');

Am I missing something?

We'll see if the rest of the site loads. Try to eliminate the issue first
By commenting it out the site loads fine.