WorldPay payments for the specific customer fail, an error is returned:
Failed to prepare redirect.
The WORLDPLAY.log shows an error similar to:
<error code="2"><![CDATA[The entity name must immediately follow the '&' in the entity reference.]]></error>
In the account details (e.g. an address) there is a symbol '&' or similar which is due to the bug #PBA-57149 ("Worldpay: special symbols are not encoded") is not encoded in the request to WorldPay.
The bug is expected to be fixed in PBA 5.5.7. As a workaround for the payments to be processed successfully for the customer, correct the customer's information (exclude the symbol that causes an issue).