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 one on github:
and now it works again!