Field values in emails causing sales links to not open

I thought this was a recent known issue, but it doesn’t seem to link up. We have a series of field values in our campaign so that as a person travels through the sequence they are directed to a different version of our sales page as needed. (So if they go back and open email #1 it will be the correct sales page for the time they are in- which changes). The problem is that we are having people opting in to join, but getting an error page at some part of the purchase process. Often this results in a double purchase. We are working on them individually but it’s frequent enough our customer support is concerned and it’s frustrating to others as well. What information can I offer to get some light on this, and when a solution can be in place?

There’s so many things this could be. An unpublished page in the process, possibly related to one type of purchase (there has been issues with both PayPal and WePay recently), the middle of the process that manages the transitions (not sure what method is being used there)…etc