Reproducing it is difficult. It does not happen everytime.

Last time it happened I had no other apps running, phone had been idle. I receive message and swipe up to get to notifications. I click whatsup notification and it shows only the glass effect background, nothing else. However, whatsup is loaded since I can long press on the screen and get input and also can awake the drop down menu from top and bottom. Noticed this because I once managed to delete conversation history...

After this happens there seems no other way out of it than restarting whatsup. You can swipe it down and click whatsup open again, bit you still get to same screen.

Could there bee something in the logs?