Unlock Bootloader from Iodé

I had installed Iodé successfully on my Fairphone 4 about 3 years ago. Now i want to flash the original Image again. But i can’t unlock the bootloader. On the command “fastboot oem unlock” i get the message:

$ fastboot oem unlock
FAILED (remote: ‘unknown command’)
fastboot: error: Command failed

How can i unlock it again?

It’s fastboot flashing unlock

Thanks!
Bootloader is now unlocked. But now i can’t unlock the critical patition. On the command fastboot flashing unlock_critical i got the message

FAILED (remote: 'Unrecognized command flashing unlock_critical')
fastboot: error: Command failed

The same on fastboot flashing get_unlock_ability :

FAILED (remote: 'Unrecognized command flashing get_unlock_ability')
fastboot: error: Command failed

are you using some really old crusty version of fastboot?

to revert to stock, best use the native instructions and ask on their forums: https://support.fairphone.com/hc/en-us/articles/18896094650513-Install-Fairphone-OS-manually

Fastboot Version is 35.02

I think it might be a faulty usb cable or usb port of the computer or the device. What happens when you type:
fastboot flashing lock_critical
does it respond on the device itself?