Only 1 w/ err "disconnected, management-protection failure"

Hello friends, I have a problem with management-protection. I use it blank and only one of my stations have this error:

Jul/26/2013 12:23:41 wireless info D4:CA:3D:8B:7D:3F@sperpop1: disconnected, management-protection failure
Jul/26/2013 12:23:42 wireless info sperpop1: data from unknown device D4:CA:3D:8B:7D:3F, sent deauth

I’ve tried some things like unmark hide ssid, increase hw. retries from 7 to 12, tx power from station and ap, upgrade RouterOS and firmware…

Anyone have a hot tip about it? Thank you so much.