Because I use the S-View cover what I do is close it and open it again and what is actually does is wake the screen. Seems like a "wake screen" bug to me, and I don't think that the proximity sensor in the settings menu can help with that, it probably works only with stock dialer and not always with third party apps like Viber. So I don't know if you try to press the power once in order to close the black screen and then open it again if it helps.
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