To summarise, valid Mastercards are being rejected. The error message is the first four digits are wrong.
I have tested three different genuine cards with the four digits 5487 and all were rejected.

Visa cards are not rejected and payment is made so I assume it is a data issue not a coding issue?

There is a subsidiary problem that the error message does not does not show the incorrect digits; just a blank.
I entered a Visa card with incorrect first four digits, it was rejected and the error message again did not show the incorrect digits