I actually had Coderus applets installed, but an old version (I didn't install anything since months, as I had no time to spend on the N9 in case of brick). It is weird it appeared out of the blue like that.

So I uninstalled T tweak (Coderus), got many errors because of permissions but eventually an "Uninstallation successful" prompt, rebooted, the applets were still here and functional, reinstalled the T tweak anyway, got again many permission errors (this time with no "success" prompt at the end of the process), and now it seems that the N9 correctly unlocks. I don't really understand but hey, it's better than before your message!

The problem is that I cannot actually update Coderus applets, but I guess I should ask in the N9QT or Coderus' threads. I won't try anything before I have time to fix the N9 in case anything gets wrong in the process.

Thanks Thedead1440, once again you saved my N9.