Il a rapidement trouvé un solution pour effectuer un hard reset et, faute de pouvoir se connecter sur swisslinux.org (mot de passe oublié et procédure de modification inopérante), il nous a transmis par courriel la solution trouvée:

If your Fairphone does not boot into Android anymore, you will have to perform the reset using the system recovery. Choose your phone below to see the relevant instructions:

    Remove any USB-C cable and turn off your Fairphone 4. If you cannot turn your device off, remove the battery for about 5 seconds, then put it back in.
    Press and hold the Volume Down button.
    Insert a USB-C cable connected to the power (can either be a power outlet or a computer).
    Release the Volume Down button as soon as you see the FAIRPHONE logo.
    Use the Volume buttons to select the option Recovery mode (on top of the screen).
    Press the Power button briefly to select.
    Wait for a couple of seconds. The FAIRPHONE logo will appear briefly. Then you will see the Android recovery menu.
    Use the Volume buttons to select the option Wipe data/factory reset.
    Press the Power button briefly to select.
    Use the Volume buttons to select the option Factory data reset.
    Press the Power button briefly to select.
    Use the Volume buttons to select the option Reboot system now.
    One last time, press the Power button briefly to select.

You are done! The first boot will take longer than usual, as the system has to initialize.

    In case you want to leave the recovery system and not continue with the Factory reset, select Reboot system now. Alternatively, you can just briefly remove the battery.

Problème [Résolu], donc.