Apple Transporter failing to upload IPA with error: Failed to create .itmsp for 'appname' status code 401

Question

I am trying to upload an ipa to Transporter (to get new version on TestFlight) and have successfully done it in the past. However, today I tried to upload and it stopped with this error popup on the screen:

Failed to create the .itmsp for 'Appname'. The web service returned an unexpected status code (401 instead of 200)

screenshot: screenshot of error

Notes:

• tried multiple times today with same issue
• App is a Xamarin app thus can't upload the other way (Xcode > Archive)
• using Transporter app released by Apple

Answer 1

Just relaunch Transporter app, or quit and r-enter it again.

Answer 2

Try completely shutting down your device and then turning it back on and trying.

I had the same issue for most of yesterday and couldn't find a solution, and just now a reboot of my MacBook made it work instantly.