How to fix "ErrorSource" when using CreateImagesFromUrls endpoint in Microsoft Custom Vision Training API

Question

When using the "CreateImagesFromUrls" endpoint of the Microsoft Cognitive Services Custom Vision Training API I get an error returned for each image saying "ErrorSource".

I am able to create tags using the API so I don't think it's an authentication issue, although it could be that the API can't load the image URL's which are images stored within SharePoint.

I have tried using a few different JSON formats for the HTTP body in the REST call.

This is an SPFX web part built using React and TypeScript. The goal is to load images from a SharePoint library and post them to the Training API. Here's my upload function:

private async uploadImages(tag: string, imageUrls: Array<any>): Promise<String> {
        const imageBatch: any = {images:[], tagIds: [tag]};
        imageUrls.map(iUrl => {
            imageBatch.images.push({url: iUrl, tagIds: [tag], regions: [
                {
                    tagId: tag,
                    left: 0.0,
                    top: 0.0,
                    width: 0.0,
                    height: 0.0
                  }
            ]});
        });
        console.log('ImageBatch:');
        console.log(imageBatch);
        //Add images
        const keyPostUrl: string = this.endPoint + 'projects/' + this.pRID + '/images/urls';
        const requestHeaders: Headers = new Headers();
        requestHeaders.append('Content-type', 'application/json');
        requestHeaders.append('Cache-Control', 'no-cache');
        requestHeaders.append('Training-key', this.trainingKey);

        const httpClientOptions: IHttpClientOptions = {
            body: JSON.stringify(imageBatch),
            headers: requestHeaders
        };

        return this.context.post(
        keyPostUrl,
        HttpClient.configurations.v1,
        httpClientOptions)
        .then((response: Response): Promise<HttpClientResponse> => {
            console.log("Sending Images.");
            return response.json();
        }).then(data => {
            console.log(data);
            console.log("Is batch successful: " + data.isBatchSuccessful); 
        });  
    }

Expected to see isBatchSuccessful = true and the images uploaded. However I get isBatchSuccessful = false, the error of "ErrorSource".

Example of response returned from API:

{
   "isBatchSuccessful":false,
   "images":[
      {
         "sourceUrl":"https://myserver.sharepoint.com/sites/dev/Vision%20Training%20List/1234/image_example_752%20-%20Copy%20(2).jpg",
         "status":"ErrorSource",
         "image":null
      },
      {
         "sourceUrl":"https://myserver.sharepoint.com/sites/dev/Vision%20Training%20List/1234/basic-image-template%20-%20Copy%20(3).png",
         "status":"ErrorSource",
         "image":null
      },
   ]
}