What version of Zen Cart and Ultimate SEO URLs are you experiencing this issue under?
Where during the checkout process is it bringing this up? Is it with a specific payment method?
What URLs are being called via AJAX by the Numinex FEC module?
Can you provide a link to the site in question so we can see exactly what is occurring?
Can you also PM a dummy customer account I can test with?
POST data will be lost if a redirect occurs. The current version of Ultimate SEO URLs does not issue a 301 redirect for any page where "POST" data is present (for security POST should always be used with form submissions during checkout).
The current version of Ultimate SEO URLs will also not rewrite pages not listed in "pages to rewrite" (unless you cleared the default list). If you cleared the list, please add back the pages you want rewritten. If you added "quick_checkout", just remove it from the list. The variable found in "main_page=" is the name of the Zen Cart "page".
You say removing "index" from the list of pages to rewrite fixes your issues? This is rather strange, as "index" refers to category pages (category_list and product_list) and the main page... Not checkout pages... Does FEC modify category pages?
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