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.