Older .NET Framework versions defaulted to use a TLS version of 1.0 or 1.1. Today, TLS 1.2 is the newest. As of 2018, Azure App Services default to TLS 1.2. This means that you might run into issues with older .NET applications failed to issue web requests against apps/apis hosted on these app services.
“The underlying connection was closed: An unexpected error occurred on a send.
This problem especially presents itself for .NET framework applications versions 4.6.1 and lower. These older applications don’t default to TLS 1.2 when issuing web requests.
There’s two ways to fix it. One is to force the .NET client code to default to TLS 1.2 (as long as the framework version supports it).
Another way is to chose minimum TLS version on a Azure App Service . As mentioned earlier, Azure App Services default to Tls 1.2 since 2018.
You can do this from the TLS/SSL settings blade in an Azure App Service.