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Hi there,
Can you confirm that your proximity sensor works properly?
Make sure that a proximity sensor is clean, and then:
Dial(phone app) *#*#7378423#*#* > Servise Tests > Proximity Switch (test it).
Keep us updated.
You have to cover the sensor and see if it says on.
I cannot figure out what could be.Usually,if it is not a screen protector or a case,it could be an antivirus or another app.
Boot in safe mode,see how it goes:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008
I suspect avast.