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

You have to agree to a new sign up in Application Loader. Select “Application Loader” under the “Xcode -> Open Developer Tool” menu (the first menu to the right of the Apple in the menu bar). Once you open Application Loader there will be a prompt to agree to new terms and then to login again into your iTunes account. After this any upload method will work.

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

cd ~  
mv .itmstransporter/ .old_itmstransporter/  
"/Applications/Xcode.app/Contents/Applications/Application Loader.app/Contents/itms/bin/iTMSTransporter"