Same issue for me. I've written to Apple using the AppStore Connect contact forms.
actnone
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Same here. It started around noon yesterday (GMT +1) for me. I'd been doing upgrades to a few libraries and had recreated certificates (using Fastlane match) while doing so, so I thought I had done something wrong, but it seems like some of the builds can be installed while others can't, and now that I see that several people here are having the same problem. Yesterday evening I pushed two different builds of the same app (but with two bundle ids, one for prod and one for testing) and I could install one of them using TestFlight, but not the other.
The errors I'm getting are:
Beta has expired
Which happens when I try to open the app (the second time) that I've successfully installed by the workaround of installing the build through the "previous builds" menu
Unable to install ""
An error occurred while installing version
Which is what I get when it fails installing or updating. The error is only displayed if I press install/update and exit the TestFlight app. Otherwise it seems to silently fail without any message.