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Hi @ruprecht,
If the other advice from the other posts you have made about this has not resolved the issue, then you will need to contact your local support team to have the phone assessed. You can find their contact information in my signature.
I recommend you contact Sony, they should be able to advise you the best steps to take next.
The link in my signature is this one:
"If you want to get in touch with the local support team for your country please visit our contact page."
Click the "contact page" link.
In my experience, I would argue Sony products all have built-in obellecence. I haven't bought Sony products for around 15 years after being disapointed by their quality. I gave them another chance recently with the Z3 Compact because they are the only company making a decently speced phone with that size screen.
I too, like you and many thousands of others (this is a VERY common defect with Sony phones) have a defective digitiser.
All you can do is get the screen replaced and never buy Sony products again. Voting with your wallet might make them consider offering products of much, MUCH, better quality the likes of those seen by Samsung or LG.
/Rant.
To double check the defect, do the following.
Got to Dialer and dial: *#*#7378423#*#*
Press: Service Tests
Press: Touch Screen
See if you can draw on upper part of the screen. If you can't, the digitiser is defective and needs replacing.