Yeah, i am following the instructions from that page. and there it is stated, that i shall do the following:
Type
adb devicesand press Enter. You will see a pop-up on your Fairphone asking if you allow USB debugging on this computer. Tap **Allow
Next, type
adb reboot bootloaderand press Enter. The phone will now reboot into fastboot mode.**.
While in fastboot mode, type
fastboot flashing unlockand press Enter. Follow the instructions that appear on your Fairphone’s screen. This action will wipe all the personal data on your phone.
I can enter the fastboot mode using Vol Down + Power, but i cannot use fastboot as it seems. It states:
./fastboot flashing unlock
< waiting for any device >
I will try to bring my adb key to the phone via sd card. That might help.