Fix a session issue in OSCommerce

General information for the website: OSCommerce based site
Kind of development: Customization of existing website
Description of requirements/features: Hi

I have an issue with my oscommerce website. I believe it is a session issue but I can't seem to correct it myself.

When a customer leaves the website to visit a payment website (paypal or sagepay) when they return, the success page is invalid, and they are returned to an empty cart and are logged out.

The error first started following a server migration. I have checked the configure.php files and cannot find an error myself, so I am not sur how to resolve the issue.

I really need this fixed ASAP

I have already used one developer to try and fix this but they failed and could not correct the issue.

The money can only be paid once the issue is fully fixed.

I can supply details for you to look into the issue before you confirm you can correct the issue.

Many Thanks
Extra notes: