Pressing "Review order" deletes all data and stops the transaction

This happens with about half the customers - literally all the fields are wiped and they have to start again... only for the same thing to happen over and over again until they give up, although I think we did have one customer who having had this problem persisted and on the sixth attempt successfully got to the review order page.

Sometimes this problem is accompanied by this comment in admin "Status: INVALID StatusDetail: 3048 : The CardNumber length is invalid." It happens too often to believe that they are all repeatedly entering too few numbers, and even if they were that shouldn't wipe the whole checkout of all their data. But more often than not there are no "Admin comments", no card length invalid messages.

The four sections at the top "Ship to", "Bill to", "Customer info", and "Payment" are populated with customer data even if there is this wiping problem, so some data is being sent to the database despite the problem. Having said that, if there are customers having their data wiped and not having an order created in admin, we might not learn about it.

The checkout form is succesfully sticky when I deliberately go on the site and don't enter some required fields - ie, I press review order and it keeps me on the page, returning what I did enter and promting the entry of data in the required fields. This wiping of data happens so often (half of all customers) that I don't think it is the form not being sticky and wiping data for customers who have not entered all the required fields. Firstly it works for me and secondly these customers have told me that on the second and third attempts they have entered all their details very carefully.

In summary, half my customers are kept at https://www.example.com/cart/checkout and can't get to the review order page.