Thank, but changing the settings like @chrisrg suggested did not help. Regarding @aibd idea: If I touch the status bar, nothing happens.
Furthermore: Flashing with /e/-recovery did not change anything. Trying to flash with TWRP (twrp-3.0.2-1-z3c.img → which is pretty old but the newest available) did not work. I get Updater process ended with ERROR: 7 “E3004: This package is for device: z3c; this device is .”. Trying to delete assert(…) in updater-script leads to "Could not find ‘META-INF/com/google/android/update-binary’ in the zip file.
Anyway, thanks all for the help.
Edit: I managed to install it with TWRP (probably some weird behaviour with Windows zipping). Anyway, no difference.