AbanteCart support - I am also having the same error show up - since Nov 15 only. Prior to that no error.
David -There are two PayPal Express Button Blcoks, you can stop the warnings by turning the BML button off and use only the Paypal Link until AbanteCart can give us a solution. I am not having any issue with the plain paypal_express_check_out_button block, only with the paypal_express_check_out bml_button