Well, problem solved.

Because I carefully installed patches one by one, in case of problems like the one I had (lockscreen unusable), I new more or less that my problem was caused by this patch : https://openrepos.net/content/zeta/p...l-notification .

So if anybody else has a problem with this patch, here is what I did :

1) go to the recovery mode as explained here : https://jolla.zendesk.com/hc/en-us/a...Recovery-Mode-
2) mount the rootfs in /mnt : "mount /dev/mmcblk1p28 /mnt"
3) chroot inside the phone os : "chroot /mnt"

Then, on the github of this patch (https://github.com/Sagittarii/sailfi...oicemail-patch) I found the modified file is "/usr/share/lipstick-jolla-home-qt5/lockscreen/Lockscreen.qml"

4) go in "/usr/share/lipstick-jolla-home-qt5/lockscreen/"
5) there are two Lockscreen.qml files : one with this name, which is the patched file, and Lockscreen.qml.webos_something else which is the previous version before applying this patch. We will restore the previous one with : "mv Lockscreen.qml.webos_something Lockscreen.qml"
6) then go out of the recoverymode as explain on the Jolla doc and reboot