What does it mean when the POST method call in C# WebForms using .NET Framework 4.7 works initially but later throws a WebException stating that the remote name 'xxx' cannot be resolved?

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This error suggests that the POST method call is trying to connect to a remote server or endpoint and is unable to resolve the hostname of the remote server. This could be due to a variety of reasons such as a network issue, DNS resolution failure, or incorrect server configuration. It could also be caused by the server being offline, the server's IP address changing or the server being moved to a different location. You may need to troubleshoot the network connectivity, DNS resolution, or server configuration to resolve the issue.