Just for
well could solve the problem but ...!
you can change the behavior at the line 501 putting something like this
'STATE' => ($order->billing['state']?$order->billing['state']:$order->billing['city']),
but then you receive another error: 10565 Merchant country unsupported.
I call paypal this is the real problem !!
paypal wpp is only implemented for US and UK.
the rest of us go on with other modules

thanks