Application Loader stuck at “Authenticating with the iTunes store” when uploading an iOS app

We have been trying to submit an app to the iTunes store using Application Loader for three days and keep getting stuck at the “Authenticating with the iTunes store” step.
We have read many forums (including stackoverflow) and tried what was suggested:

• making a new provisioning profile
• using different or multiple versions of Application Loader
• changing proxy settings
• rebooting the Mac
• uploading at a different time of the day, etc.

We have even left it running overnight and have not had success with getting past this step. Unfortunately, no feedback is given about what the issue may be, and we have not gotten any error messages. We have submitted multiple apps without any difficulty in the past but are completely stuck this time!
How were you able to solve it?

This only started happening to me today (May 2017) and no answers in this thread solved my issue. The resolution for me was from here;

https://forums.developer.apple.com/thread/76803

Open Terminal. Change to home directory,

cd ~  

Move the current transporter directory,

mv .itmstransporter/ .old_itmstransporter/ 

Invoke the following file to let Transporter update itself.

"/Applications/Xcode.app/Contents/Applications/Application Loader.app/Contents/itms/bin/iTMSTransporter"

Wait till it updates, then open Xcode and attempt upload.