I had this problem too and just managed to fix it. I run with a transparent panel - a blank grey sqaure for no good reason is irritating.
Having fixed it I can't now find the link that gave me the clue or post the output of the commands, but try this -
1] Fire up a terminal
$ ps ax | grep pam | grep icon
3] Hopefully you'll get one process listed. You'll want the number at the start of that line, that's the PID, replace the PID in the next command with that number.