Microsoft has recently disabled support of TLS 1.0 on Azure App Services and switched to TLS 1.2 by default (see TLS plans for Azure App Service discussion). Technical details can be found here: TLS best practices with the .NET Framework.
Any .Net call relying on TLS 1.0 is leading to authentication issues on Azure App Services:
Exception
System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. --->
System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. --->
System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host.
Note: That does not affect Sitecore 9 deployments.
You may also be interested in the following article: TLS 1.0 and 1.1 are deprecated for use with Azure Cache for Redis.