GetUser API Request Returns Authentication Error: 105, InvalidCredentials, `Authentication failed. Either supplied credentials are invalid or the account is inactive`

I am currently experiencing an issue with the GetUser request in the Microsoft Advertising API. Please find the details below:

Problem Overview

When making a GetUser request, an InvalidCredentials error (error code 105) is returned.

Error Message

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
    <s:Body>
        <s:Fault>
            <faultcode>s:Server</faultcode>
            <faultstring>Invalid client data. Check the SOAP fault details for more information. TrackingId: b4e84d0e-f652-4d1d-80db-4506a35cf1f5.</faultstring>
            <detail>
                <AdApiFaultDetail xmlns="https://adapi.microsoft.com" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
                    <TrackingId>b4e84d0e-f652-4d1d-80db-4506a35cf1f5</TrackingId>
                    <Errors>
                        <AdApiError>
                            <Code>105</Code>
                            <Detail i:nil="true"/>
                            <ErrorCode>InvalidCredentials</ErrorCode>
                            <Message>Authentication failed. Either supplied credentials are invalid or the account is inactive</Message>
                        </AdApiError>
                    </Errors>
                </AdApiFaultDetail>
            </detail>
        </s:Fault>
    </s:Body>
</s:Envelope>

Verified Information

1. The user authenticated via OAuth has confirmed access to the Microsoft Advertising account.
2. The Advertising account being used is not in a sandbox environment.
3. The access token is valid (the error occurs even immediately after refreshing).
4. The correct Developer token is being used.
5. The Advertising account currently in use has not completed identity verification, but we have confirmed that GetUser requests for other Advertising accounts will be correctly responded to even if identity verification has not been completed.
6. I am making the request following the guidelines in this Document

Questions

1. Given the above situation, why is the GetUser request failing?
2. Are there any additional steps we should take to resolve this issue?

Your review and response would be greatly appreciated.