Thanks for your very fast replies. Some more info:

@Kuroi: The module works in Zen Cart (I have had it installed for months without any problem); so it should not be a source issue.

@ebusinessman: Thanks for the help too. The files you mention are exactly as you say. I have not added the code you mention, because I do not know what to add.

Some more info:

1.- I have compared all files of the module currently on the server with an old backup (when it worked)... and there are no differences on the code.

2.- The bank uploaded a personalization of the pages in their server to match mines. However, I do not think that this can be the issue, because when customers pay and return the cart is emptied. It is only "reloaded" when the customer logs in; this makes me thing that the info on the bank server has nothing to do with the problem (...but I may be wrong).

Are there any other files dealing with the cart and sessions?

Desperate is a soft word to express how I feel. +