This is Lana from Shopify.
I see you have already tried a number of things to look into this issue, so I investigated the error code a little further on my side. The error code 10009 can be produced due to a number of different issues:
This screenshot comes from the PayPal Help Centre, where they have an article about this error code.
Did you make sure to mention this error to PayPal when you contacted them? If not, please do so as they might be able to pinpoint which one of the above scenarios produced the error code for you. Because PayPal is a third party gateway, we are unable to assist with all of the troubleshooting involved - but if PayPal support asks for some additional info on the Shopify side of things, you can feel free to reach back out to me here and I will see how I can help you out.