Get connection was cloesed error during PDF merge at deployed WCF service

HI, 

I am using Adobe tools api for combining PDFs to single file. The code is very stragihgt forward. 

It is a .NET wcf web service. however, I got the following error. 

Response received from the request: The underlying connection was closed: An unexpected error occurred on a send. 

Any idea?

Thanks. 

Here is the code:

  Credentials credentials = Credentials.ServiceAccountCredentialsBuilder()

                               .FromFile(adobelicen + "/pdftools-api-credentials.json")

                               .Build();

                    ExecutionContext executionContext = ExecutionContext.Create(credentials);

                    CombineFilesOperation combineFilesOperation = CombineFilesOperation.CreateNew();

                    String[] files = source.Split(',');

                    FileRef firstFileToCombine = FileRef.CreateFromLocalFile(files[0]);

                    PageRanges pageRangesForFirstFile = GetPageRangeForPDF();

                    // Add the first file as input to the operation, along with its page range.

                    combineFilesOperation.AddInput(firstFileToCombine);

                    //combineFilesOperation.AddInput(firstFileToCombine, pageRangesForFirstFile);

                    // Create a second FileRef instance using a local file.

                    FileRef secondFileToCombine = FileRef.CreateFromLocalFile(files[1]);

                    PageRanges pageRangesForSecondFile = GetPageRangeForPDF();

                    combineFilesOperation.AddInput(secondFileToCombine);

                    /*FileRef thridFileToCombine = FileRef.CreateFromLocalFile(@"aerial.pdf");

                    combineFilesOperation.AddInput(thridFileToCombine);*/

                    // Execute the operation.

                    FileRef result = combineFilesOperation.Execute(executionContext);

                    // Save the result to the specified location.

                    result.SaveAs(output);