Is everything working well before and suddenly the express checkout stopped working then check for below.
Did you recently shift to HTTPS and the Form action is still on HTTP? If yes you just need to change the form action to have HTTPS.
Also see if you are passing order total in request API. The complete error description is as follows.
10400 DoReferenceTransaction SHORT Transaction refused because of an invalid argument. See additional error messages for details.
LONG Order total is missing.
Note: TotalOrder amount is missing.