Test purchases no longer work on Android

I have an app using IAP, released using 5.5.1, but it does no longer work if rebuilt using 5.5.1, 5.6.1 or 5.7.0.

This was strange so I decided to try the QtHangman example. I only edited the package name and IAP product identifiers (and created a keystore certificate). Built using Qt 5.7.0, Android build SDK = 24, minimum SDK = 16 and target SDK = 16

The in-app identifiers was added to Google Play. Beta and Production tested.

The error message I get when trying to purchase a non-consumable product in the app is: "The item you requested is not available for purchase"

And the log says:
D libqthangman.so: qrc:/qml/qthangman/StoreView.qml:110 (onPurchaseFailed): qml: my.packagename.unlockvowels purchase failed
D libqthangman.so: qrc:/qml/qthangman/StoreView.qml:112 (onPurchaseFailed): qml: Item unavailable

Tested on Android 6.0 and 4.4.4.

Same code works great on iOS.