Share your experience!
While the phone is blocked it's sometimes turns on the backlighting,changed the walpapers,rebooted the phone.
It appeared after updating to the latest update.
I suggest that you try to run a software reinstall using SUS.
http://www.sonymobile.com/gb/tools/update-service/
If the problem still remains you should contact your local support team, to have the phone examined and fixed at an repair center.
http://www.sonymobile.com/global-en/support/contact-us/contact-info/
