TestFlight error "Your request couldn't be completed"?

Question

I'm testing an iOS App using TestFlight. I have already successfully tested 8 Build pre-release but now it is not working anymore.
I have upload the built and it has been approved by Apple, invitations to external testers has been sent and properly received, but when they open TestFlight and try to download the App the download stops just before the end and an alertView says:

Your request couldn't be completed

NOTES: I have tried the download on different devices but none has worked.
TestFlight is incrementing installation count also if none download has been completed.
I have upload the built today in the morning.

Answer 1

Found solution.

Latest XCode version changed the default build code signing identity to Developer, when it should be Distribution to work with TestFlight. While it throw any error while uploading, it's the one causing the problem.

Just change it and it will work again