I am having some problems with interstitials on some devices. The interstitials are showing correctly, but when the user presses close, the screen turns black and the only solution is to close the app. This does not, however, happen in all devices. In my test device (android 6) and in the emulators, it works fine. However, some users with KitKat report this problem.
Edit: In the phone of a user (friend of mine), the screen does not go black. The interstitial closes but the app freezes (except the banner on the bottom, that continues to work).