Temporary grey/black screen when unlocking phone

[The_Dave]

Hi,

Once or twice a day when I press the finger print button to unlock my (already turned on) Z5 Compact I see a dark grey blank screen. I have to press the button to switch it back to stand-by and then unlock it again four or five times before the grey screen goes away and I can use my phone again. It's particularly frustrating when it stops me from answering calls.

Key facts:

 - It only happens when unlocking the phone by pressing the power button, or when I have incoming calls which activate the phone

 - It seems like a software issue, but it even happend once during set-up after a full software repair with Xperia Companion (which I think should rule out pretty much any software fix?)

Is there anything else I can do?

Thanks

Dave