aws-sdk-net | official AWS SDK for .NET | AWS library

So the issue was in IIS.

I was running the Application Pool as the user that needs to be authenticated, which is correct. But you also need to set the following:

Application Pool -> Advanced Settings -> Process Model -> Load User Profile = True

Issue fixed with versions AWSSDK.Core 3.3.107.16 AWSSDK.S3 3.3.111.17

Endpoint Discovery is the process where a specific URL (the "discovery endpoint") is accessed, which returns a directory of endpoints for using the system. Other code can interrogate that directory to find the specific URLs for accessing various resources. This allows the service host to change its URLs as needed without interfering with their client's ability to use those services.

From Google's API docs:

The Discovery Document describes the surface for a particular version of an API. The information provided by the discovery document includes API-level properties such as an API description, resource schemas, authentication scopes, and methods.

The discovery document is usually JSON or XML, and can have any structure. Once created, the discovery document's struture can't change, otherwise it would interfere with clients. Only the values of the endpoints can be updated.

Using Endpoint Discovery is usually optional, but you should be aware that service endpoints can change over time, breaking your code. Using Endpoint Discovery means that your code will automatically update to the new endpoints.