For the past 24 hours my TestFlight alpha testers have been unable to update or launch my app. It seems Apple has an issue: App won't install from TestFlight | Apple Developer Forums.
There is a workaround. If you get your testers to go back to a previous build in TestFlight from a week (or more) ago, and install that one, then they can go in and install the latest build.
Just in case it helps anyone and saves you some time. 