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Update:
I've been talking to Sony Support and they have no idea at this moment to what can cause the fault.
I explained what I've done myself (thanks to you guys) as to finding the fault and that was the only things the guys on the phone could suggest as well.
He's been writing down eveything about the issue and I've sent him an e-mail with screenshots and what I found out just after I talked to him, which is:
Since I have the tracking of touch and show touch activated there is in the top of the screen some indicators to where the touch is registred, how hard is being pressed and so. I've found out that the area where it is faulty is the same, whether horizontally or vertically and regardless being in apps, settings, home screen or whatever.
It all comes down to an area between ca 75-265 on the Y-axis when upright/vertical and same ca 75-265 on the X-axis when on the side/horizontal, and then all across the screen. When moving my finger in between that area it detects touch on both sides (as seen on previous screenshots) and the axis counter stops counting in between as well.
Furthermore, moving my finger to the top when phone is upright, and moving my finger to the left when on the side, it has a hard time registerering my finger from ca 40 and up/out to 0. It stops counting/registering movement untill my finger is out of the screen and then (of course) detects that the screen is not touched.
The support-guy and I came to the conclusion that I should send the phone in for repair and give the service center the same description as here and what I've done myself and see if they can figure out what's going on.
When (hopefully) the problem has been solved I post the solution here and send it to the support-guy as well so if anybody else experience this (I really don't hope so) there can be an ansver to it...
Thank your for the update @Celica_85, I'm sorry there wasn't an easier way to resolve this. When/If you decide to have your phone sent to repairs, make sure to provide a descroption as detailed as possible.
It'd be great if you keep us updated about your case.