Thank you for the answer.
I tried to downgrade to 11, but even though sticking to the process step by step - when I press Enable OEM - the system requires the phone passcode and then DOES NOT require any unlocking code, only says that OEM is active. when restarting, the bootloader is still locked - thus not able to reflash it
any advice?
I assume, you only did the things in part 1 up to “Step 2. Enable OEM unlocking” by enabling “OEM unlocking”.
The second line in the developer settings says “Allow the bootloader to be unlocked”.
So switching on this toggle doesn’t really do much, unless you continue with “Part 2: On your computer (Windows and macOS)” of https://support.fairphone.com/hc/en-us/articles/10492476238865 (./fastboot flashing unlock and ./fastboot flashing unlock_critical commands).
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OEM unlocking and bootloader unlocking isn’t the same. After you did the OEM unlock, you have restart your phone into fastbood mode as described here in part 2: https://support.fairphone.com/hc/en-us/articles/10492476238865-Manage-the-Bootloader
Then you can unlock the bootloader with the command fastboot flashing unlock (this will also wipe your phone)
EDIT: too late 
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