Check your bulbs, I received that message when one of my taillights died. This can also mean that your brake light switch needs replacement. I had this error once every 2 or 3 months, then I bought the switch (it's cheap, under 20.00 I think). The light hasn't come on since even though I've not installed it yet.
The instructions are here:
http://www.bmwe34.net/e34main/mainte...ake_switch.htm