We have hosted our storage on AmazonS3 so you are getting the exception from AmazonS3.
I get this error even though my Azure storage account is set to be the default.
Here’s the code (C#):
    static void AsposeTest()
    {
        var CLIENT_ID = "<redacted>";
        var CLIENT_SECRET = "<redacted>";
        var BASEPATH = "https://api.aspose.cloud";
        var AUTHPATH = "https://api.aspose.cloud";
        IStorageApi storageApi = new StorageApi(CLIENT_ID, CLIENT_SECRET, BASEPATH, AUTHPATH);
        var x = storageApi.StorageExists("davetest");
        var y = storageApi.StorageExists("cubiksdavetest");
        Console.WriteLine($"davetest exists: {x}. cubiksdavetest exists: {y}");
        var sourceUrl = "https://reporting-delivery-service.azurewebsites.net/report";
        var width = 800;
        var height = 1200;
        var leftMargin = 15;
        var rightMargin = 15;
        var topMargin = 15;
        var bottomMargin = 15;
        var fileName = "HTMLPage1.html";
        var outPath = @"c:\temp";
        IConversionApi convApi = new HtmlApi(CLIENT_ID, CLIENT_SECRET, BASEPATH, AUTHPATH);
        try
        {
            var response = convApi.GetConvertDocumentToPdfByUrl(
                sourceUrl, width, height,
                leftMargin, rightMargin, topMargin, bottomMargin);
            if (response?.ContentStream != null)
            {
                var stream = response.ContentStream;
                var outFile = Path.Combine(outPath, response.FileName);
                if (!Directory.Exists(outPath)) Directory.CreateDirectory(outPath);
                using var fstr = new FileStream(outFile, FileMode.Create, FileAccess.Write);
                stream.CopyTo(fstr);
                fstr.Flush();
                Console.WriteLine($"GetConvertDocumentToPdfByUrl Succeeded ({response.Status}): result file saved to {outFile}");
            }
            else
            {
                Console.WriteLine("Empty response");
            }
        }
        catch (Exception e)
        {
            Console.WriteLine($"Exception from GetConvertDocumentToPdfByUrl: {e}");
            throw;
        }
        try
        {
            var response = convApi.GetConvertDocumentToPdf(
                fileName, width, height,
                leftMargin, rightMargin, topMargin, bottomMargin,
                null, null);
            if (response?.ContentStream != null)
            {
                var stream = response.ContentStream;
                var outFile = Path.Combine(outPath, response.FileName);
                if (!Directory.Exists(outPath)) Directory.CreateDirectory(outPath);
                using var fstr = new FileStream(outFile, FileMode.Create, FileAccess.Write);
                stream.CopyTo(fstr);
                fstr.Flush();
                Console.WriteLine($"GetConvertDocumentToPdf Succeeded: result file saved to {outFile}");
            }
            else
            {
                Console.WriteLine("Empty response");
            }
        }
        catch (Exception e)
        {
            Console.WriteLine($"Exception from GetConvertDocumentToPdf: {e}");
        }
        try
        {
            var response = convApi.PostConvertDocumentToPdf(fileName, $"report.pdf", width, height, leftMargin, rightMargin, topMargin, bottomMargin, null);
            if (response != null)
            {
                Console.WriteLine($"PostConvertDocumentToPdf Succeeded ({response.Status}): Result file uploaded as report.pdf");
            }
        }
        catch (Exception e)
        {
            Console.WriteLine($"Exception from PostConvertDocumentToPdf: {e}");
        }
    }
The output of this code is:
davetest exists: False. cubiksdavetest exists: False
GetConvertDocumentToPdfByUrl Succeeded (OK): result file saved to c:\temp\report_converted.pdf
Exception from GetConvertDocumentToPdf: Aspose.Html.Cloud.Sdk.Client.ApiException: Error calling GetConvertDocumentToPdf: StatusCode=500 (InternalServerError); Internal Server Error
   at Aspose.Html.Cloud.Sdk.Api.Internal.ApiImplBase.CallGetApi(String path, Dictionary`2 queryParams, String methodName) in C:\Source\Samples\aspose-html-cloud-dotnet\Aspose.HTML-Cloud\Api\Internal\ApiImplBase.cs:line 69
   at Aspose.Html.Cloud.Sdk.Api.Internal.ConversionApiImpl.GetConvertDocumentToPdf(String name, Nullable`1 width, Nullable`1 height, Nullable`1 leftMargin, Nullable`1 rightMargin, Nullable`1 topMargin, Nullable`1 bottomMargin, String folder, String storage) in C:\Source\Samples\aspose-html-cloud-dotnet\Aspose.HTML-Cloud\Api\Internal\ConversionApiImpl.cs:line 174
   at Aspose.Html.Cloud.Sdk.Api.HtmlApi.GetConvertDocumentToPdf(String name, Nullable`1 width, Nullable`1 height, Nullable`1 leftMargin, Nullable`1 rightMargin, Nullable`1 topMargin, Nullable`1 bottomMargin, String folder, String storage) in C:\Source\Samples\aspose-html-cloud-dotnet\Aspose.HTML-Cloud\Api\HtmlApi.cs:line 413
   at HtmlToPdfTest.Program.AsposeTest() in C:\Source\NexusReporting\HtmlToPdfTest\Program.cs:line 179
PostConvertDocumentToPdf Succeeded (NoContent): Result file uploaded as report.pdf
The PDF output from GetConvertDocumentToPdfByUrl is a 0 length file.