Pay and then redirect to a webpage

[Claireonthewww]

Hi everyone

I have PayPal on my website and once someone has paid it should redirect the customer to a web page to allow them to submit a document.  From my website point of view it all works perfectly - and when it was tested using Sandbox it all worked.  However, not its live for payments, once people have paid they are getting sent to a 404 error page, not the page they should be.  Is it a setting in PayPal I need to change?

Any help would be greatly appreciated.