UploadSiteHostedPictures fails with error 5

[soundman333222]

I've found many questions that contain this issue, but I'm unable to find a fix.

I am able to do other things, I can get a application token, and a user token.

I'm trying to upload an image via the UploadSiteHostedPicture XML endpoint.

This is my test code:

 private static async Task Main(string[] args)
 {
     var userToken = "<userToken>";
     var client = new HttpClient();

     var xmlContent = new StringContent(
             "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
             "<UploadSiteHostedPicturesRequest xmlns =\"urnapis:eBLBaseComponents\">" +
             "<ExternalPictureURL>https://i.ebayimg.com/thumbs/images/g/h~kAAOSwehxmAqF7/s-l500.webp</ExternalPictureURL>" +
             "<WarningLevel>High</WarningLevel>" +
             "</UploadSiteHostedPicturesRequest>", Encoding.UTF8, "application/xml");
     var content = new MultipartFormDataContent();
     content.Add(xmlContent, "XML PAYLOAD");

     var request = new HttpRequestMessage(HttpMethod.Post, "https://api.sandbox.ebay.com/ws/api.dll");
     request.Headers.Add("X-EBAY-API-APP-NAME", "AppName");
     request.Headers.Add("X-EBAY-API-CALL-NAME", "UploadSiteHostedPictures");
     request.Headers.Add("X-EBAY-API-CERT-NAME", "CertName");
     request.Headers.Add("X-EBAY-API-COMPATIBILITY-LEVEL", "1371");
     request.Headers.Add("X-EBAY-API-DETAIL-LEVEL", "0");
     request.Headers.Add("X-EBAY-API-DEV-NAME", "DevName");
     request.Headers.Add("X-EBAY-API-IAF-TOKEN", userToken);
     request.Headers.Add("X-EBAY-API-RESPONSE-ENCODING", "XML");
     request.Headers.Add("X-EBAY-API-SITEID", "3");
     request.Content = content;
     var response = await client.SendAsync(request);
     response.EnsureSuccessStatusCode();
     Console.WriteLine(await response.Content.ReadAsStringAsync());
 }

The response is always the same:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<UploadSiteHostedPicturesResponse xmlns="urnapis:eBLBaseComponents">
    <Timestamp>2024-07-23T21:46:44.648Z</Timestamp>
    <Ack>Failure</Ack>
    <Errors>
        <ShortMessage>XML Parse error.</ShortMessage>
        <LongMessage>XML Error Text: "{0}".</LongMessage>
        <ErrorCode>5</ErrorCode>
        <SeverityCode>Error</SeverityCode>
        <ErrorParameters ParamID="0"/>
        <ErrorClassification>RequestError</ErrorClassification>
    </Errors>
    <Version>0</Version>
    <Build>mediasvcs-5.0.33_20240718171503670</Build>
    <PictureSystemVersion>2</PictureSystemVersion>
</UploadSiteHostedPicturesResponse>