PayPal Express has stopped working on my MarketPress site

PayPal Express has stopped working. Error message cites several "invalid arguments" relating to totals (see below).
There was a problem connecting to PayPal to setup your purchase. Please try again.
10401 - Transaction refused because of an invalid argument. See additional error messages for details. - Order total is invalid.
10429 - Transaction refused because of an invalid argument. See additional error messages for details. - Tax total is invalid.
10413 - Transaction refused because of an invalid argument. See additional error messages for details. - The totals of the cart item amounts do not match order amounts.

I haven't changed anything since the last successful sale.