Not possible to boot into TWRP recovery -- upgrade LG H850

I’m not able to boot into TWRP recovery.

I followed the user general upgrade guide which is a general description of the device specific upgrade guide for H850.

The device is on eOS 0.14 Android 7.1.2. I had flashed TWRP before and it appeared regularly, when updating the eOS OTA. It does not matter now, whether I flash a new TWRP or try to use the TWRP which may be baked into the device. Steps 5., 6. and 7. in manual. Step 8. is impossible to get through. I’m not able to get into TWRP.

  1. command line variant
fastboot boot twrp-x.x.x.img

does not work:

booting FAILED (remote: 'unknown command')
fastboot: error: Command failed
  1. Manual variant
    Pressing volume button as described in Install manual, step 6 leads to Factory data reset: Delete all user data: yes/no, which is not documented in manuals. :roll_eyes: Shall I go for it and delete the user data first? Did anybody go this way and got to TWRP successfully? :anguished:

I’m at the end with nerves pressing buttons. There is willpower, but there are also unfortunate quantum effects.

Regain your privacy! Adopt /e/ the unGoogled mobile OS and online servicesphone

OK, I went for the Delete all data: YES and YES and entered the TWRP. I finished the upgrade manual steps 9. to 14. I received the following messages:

At the command-line of the Windows:

Total xfer: 1.00x

At the TWRP:

zip signature verification failed

Because I did not supply and verification key, I expected the message. Rebooting the devices brought the device into Your device software can't be checked for corruption screen and blue LED slow flashing. This screen appeared after every reboot. But then the /e/ screen appeared. No /e/ loading screen now. Well… What did I miss? What shall I do now to get (any) proper eOS?

Well, this is a continuation of my monologue: I installed the zip image of the eOS from the SD card in TWRP successfully.