@ titan, I think I have another problem than you with my xperia Z1, but thanks anyway.
@ piper, I did a test-installation with KDE and installed kio-mtp before but still the telephone is not recognized. I shall read and check your links later when I have more time.
One thing more.... I tried to flush down the telephone in the toilet so I can by a new one but it didn't work, and after bringing the phone up from the water it's still working so I have to stick with it for a while....