thanks for your answer. You are talking about iOS or this is happening to you on android also?? because i tested on android and i didnt have problems but maybe i missed something out.
edit: i can confirm that this is also happening to me on iOS for heyzap video interstitials and heyzap rewarded videos. On the other hand heyzap static interstitials are working fine for me sometimes , i showed 5 ads and two of them caused the issue... i've just re-tested on android and on my android device( version 4.4.2 ) chartboost, adcolony,heyzap,unity rewarded video ads are working fine. I will re-test regular interstitials on android.
edit2: i guess we can't use it for iOS at the moment since heyzap ads freeze the screen and we can't disable heyzap ads...