Gradually, web servers switch to new security protocols and delete security protocols that are not recommended, so this may be the cause of your error. Usually, .net will automatically find common security protocols, but sometimes you need to use ServicePointManager.SecurityProtocol performs display update.You can try the following code to force the latest protocol type.
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls13;
About SecurityProtocolType.
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