Thank you for your answer. I ended up flashing a stock SONY image using newflasher on linux.
Unfortunately my phone has a network problem that appears in Android and thus SFOS. It all started with the phone becoming very hot and after that the network disappeared. It sometimes comes back a little while, like today, but then disappears again to not come back anymore.
I will try to open the phone and eventually change the antenna frame...
None of you ever had this and knows better? (sorry for the off-topic)