Anyhow I don't use any case or cover.
I will return the phone and get my money back. I'm very sorry about that because I have been waiting for the phone to replace my old Xperia ray for nearly one year.
This touchscreen bug has been discovered on Xperia Z and Z1 and z1s, now the same error applies to the z1c. Sorry Sony, I will not buy an Xperia phone anymore.