I can not add an event to the android checkin app with indico 2.0a1 (i have not tried any other version).
After scanning the qr-code, a new window with a navigation bar on top opens.
The url in the bar contains &redirect_uri=http%3A%2F%2Flocalhost%2F
in Administration -> Integration -> Applications -> Checkin App
"Allowed authorization callback URLs" is not customizable (it is grayed out)
The http://localhost URI is correct; it’s a mobile app after all that does not run on a specific host/URL.
Could you try deleting all app data/cache? That fixed the problem for someone else who reported it on github recently? Reinstalling the app might help too since that’ll also erase all app data.