We're also blocked from testing new app builds since yesterday because of this issue. My iPad makes the following requests to testflight.apple.com:
/v1/session/authenticate (200 OK)
/v3/accounts/***/apps (200 OK)
/v1/devices/apns (200 OK)
/v2/accounts/***/apps/***/builds/*** (500 INTERNAL SERVER ERROR)
The body in the response is { "data": null, "error": null}