This is most likely a 'mod/plugin' that is causing your trouble or other sites would have reported it. I would be looking at what commands were being performed in the logs prior to this being triggered. The first step in solving a problem like this is to be able to repeat the condition.
I was going to suggest a method to trap the error and use xdebug to trace through it once you could repeat the condition, but that method might be too advanced of a step for you -- after reading through you threads. If I am wrong please let me know.
Since your a production board, disable any PM mods you are using for now and see if the problem goes away. It would be helpful if you posted what you tried.