eCart 6.0.5 with PayPal Express (UK) transaction failure

I am using eCart 6.0.5 with PayPal Express checkout. I am able to successfully cary-out transactions using my PayPal account and when paying by card from my location here in France. However, I have several people testing the site for me in the UK who say that they are unable to complete a transaction using either their PayPal account or when using a card. What happens is at the last step on PayPal they are returned to the cart page which shows empty, not the error page or success page. We are testing using live transactions not a sandbox. The weird thing is I talked someone through this in the UK and it failed as described but on a second try it worked. Does anyone know what could be causing this to happen?

the problem is domain switching.

it happens when the address goes between using www and non www for the domain.

for example:

if a user starts browsing your site using the address:
http://mydomain.com/store.php

and then paypal returns them to the www address
http://www.mydomain.com/pp_confirm.php

those are considered 2 separate domains and thus 2 separate browsing sessions. The items added to the cart in the non www address no longer exist when returned to the address using www


to prevent this, use mod_rewrite to force all traffic to use the www address:
http://www.htaccessbasics.com/force-www-nonwww-domain/

Thanks Jason I have now applied this and hope to get some feedback next week from the folks who had the issues, so fingers crossed.