Install /e/ on a Fairphone FP4 - “FP4” (beta)

To all of you, who successfully installed /e/. I tried to follow the guide given in the initial post.
I’m failing at unlocking the bootloader.

  1. Reboot on fastboot by running the command in the PC console adb reboot bootloader

This is the last step that is working.
This one is not working:

  1. Ask for unlock with fastboot flashing unlock

I only get " <waiting for any device" >

adb doesn’t recognize the device anymore once in bootloader mode. What am I missing?