the bug came after the upgrade of 1.7.6 to 8.1.2.
looking at the paypal payment process, you see screenshot 1 (at the bottom).
when i try to create an invoice or to change the status of the order, the message is seen in screenshot 2 (in the middle).
on trying to create an invoice for the order with bank wirement, you see screenshot 3 (the upper one).