My cardboard app does not work with Android 4.2 or below -- hangs at splash screen

My son wrote a cardboard app using Unity and put it on the Google play store.

https://play.google.com/store/apps/details?id=com.bobinthehob.randomrelax&hl=en_GB

Works fine on my Android 4.4 phone; but on his phone (Android 4.1) we just get the unity splash screen and then nothing happens forever. I tested it on some emulators and it seems to me that it works on 4.3 and above, but not on 4.2. So we set the min SDK to 4.3 (which means that people with 4.1 devices now can't even see the app on Google Play of course), but I am interested in finding out what's going wrong. My son has written other cardboard apps in the past which have worked fine on his phone. Obviously I've not given enough information for people to start debugging this if it is a new issue, but are there any seasoned veterans who can guess what we might have done wrong?