I have been fighting for the same problem for several hours; After writing Checkout PayPal Express integration using a SOAP API with a signature more than a year ago, it was put in the background (all unit tests running at that time). Today I return to the error:
Failed to set secure channel for SSL / TLS with authority "api-3t.sandbox.paypal.com"
After much research, I highlighted the problem in our instance to ServicePointManager.SecurityProtocol by default, ServicePointManager.SecurityProtocol , updating this to SecurityProtocolType.Tls12 , fixed the problem.
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
Anh12
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