Just as a follow up for anyone still running into this – I had added the ServicePointManager.SecurityProfile options as noted in the solution:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
And yet I continued to get the same “The request was aborted: Could not create SSL/TLS secure channel” error. I was attempting to connect to some older voice servers with HTTPS SOAP API interfaces (i.e. voice mail, IP phone systems etc… installed years ago). These only support SSL3 connections as they were last updated years ago.
One would think including SSl3 in the list of SecurityProtocols would do the trick here, but it didn’t. The only way I could force the connection was to include ONLY the Ssl3 protocol and no others:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
Then the connection goes through – seems like a bug to me but this didn’t start throwing errors until recently on tools I provide for these servers that have been out there for years – I believe Microsoft has started rolling out system changes that have updated this behavior to force TLS connections unless there is no other alternative.
Anyway – if you’re still running into this against some old sites/servers, it’s worth giving it a try.