How remove blank white screen after unlocking
I have had your same problem these days, my mom's phone (your same model) was full with pop ups ad. This kind of problem are caused by spyware/malware apps. I did a hard reset, but nothing.

I looked in the preinstalled apps (Settings -> Apps -> ALL) for something strange. I found two Download Manager apps. I disinstalled the one without the icon. This resolved all my issues. Hope can help you because for me was really annoying.