I just wanted to thank you all! The Paypal Express button in my shop stopped working September 1st, 2020. The clients saw a "You not have permission to access this resource" when clicking on the button. When I tried from the Admin - modules -  Payments - Paypal Checkout Express the "Test API Server Connection" it couldn't connect. My SSL certificate is with DigiCert Inc. Sure enough, the /modules/payment/paypal/paypal.com.crt did not include all the certificates. Changed it with the on