Lock screen

On Aramo, my screen is locked after some inactivity. Sometimes (it just happened, I already saw that but can't remember when), after typing my password to unlock, instead of my session, I have a locker and a message "This session is locked" and under that "You will be redirected automatically to the unlock windo in a few seconds" and then nothing happens (I translate from French, wording might not be that in English). I switched to a terminal and logged in, then "ps -ef |grep lock", there is a light-locker process, killing that process solves the problem.

Any idea why this is happening?