Just to close this out. Now i have flashed 3.1.1 using the manual way. After the flash finished, the phone reboots automatically. The boot process will pause at the ‘Your bootloader ist not locked’ screen for some seconds.
Here press volume down and power and keep it pressed until the phone reboots again into fastboot. This must be done, before the phone boots for the first time.
Then finally:
fastboot flashing get_unlock_ability
< waiting for any device >
(bootloader) get_unlock_ability: 1
OKAY [ 0.001s]
Finished. Total time: 0.001s
and the bootloader can be locked (see e/os documentation ‘manual install’ how to do this.
Oliver