I had a similar problem with my 2000 Z3/2.3 roadster.
The mechanic I took it to (not dealer) said the seat belt sensor was bad. He said that in California, it was not legal to fix seatbelt assemblies. So he replaced the socket that that seatbelt (driver side) plugs into. That did the trick.
Now I have the same problem on the passenger side.
BTW. I was told that when this light is on, that airbag will not deploy.
FWIW...