1.3.8 too many redirects at checkout when using FEC addon

My shop is on version 1.3.8 and has FEC one page checkout.

Over many months many customers at our shop have complained they can't checkout and getting "Too many redirects" or "Timeout" errors.
These customers report trying different browsers and getting similar errors.

I have been unable to replicate the issue.

If it is the FEC can you advise of a way I could test that.

In the access logs... I see several runs of logs like this which seem to point to the issue. I can't see any related issues in the error logs .

XXX.202.142.215 - - [28/Feb/2012:01:01:43 -0800] "GET /shop/index.php?main_page=checkout HTTP/1.1" 302 - "https://www.myshop.com/shop/index.php?main_page=shopping_cart" "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.19) Gecko/2010031422 Firefox/3.0.19 (.NET CLR 3.5.30729)" 1350 623
XXX.202.142.215 - - [28/Feb/2012:01:01:44 -0800] "GET /shop/index.php?main_page=checkout&action=null HTTP/1.1" 302 - "https://www.myshop.com/shop/index.php?main_page=shopping_cart" "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.19) Gecko/2010031422 Firefox/3.0.19 (.NET CLR 3.5.30729)" 901 485
XXX.202.142.215 - - [28/Feb/2012:01:01:44 -0800] "GET /shop/index.php?main_page=checkout&action=null HTTP/1.1" 302 - "https://www.myshop.com/shop/index.php?main_page=shopping_cart" "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.19) Gecko/2010031422 Firefox/3.0.19 (.NET CLR 3.5.30729)" 901 485

Any guidance would be much appreciated.

Thank you.

It was these kinds of problems that led me to create the Zen Cart Notifier Trace add-on. It's a pretty simple install; then you can see exactly which modules are causing the redirect loop on your site.

Those logs definitely point to FEC troubles.

It could also just be that your v1.3.8 site has been damaged by intruders, based on its well-known security vulnerabilities. You should be upgrading ASAP.