There are a whole mess of things it might be, so I'll throw in some assumptions followed by my 2c worth.
If the light is staying on strongly after startup my first port of call would be the brushes, check to see if they are worn down, and if not worn down, whether or not they are sticking. That white plastic gets terribly fragile with age and as a BMW part it will make your eyes water. The good news is that as far as Bosch are concerned, a brush holder is a brush holder is a brush holder, your local auto electrician should be able to supply you with a brand new Bosch brush holder for under $20.
If you need new brushes try and find a (non-genuine) set that has little flat terminals on the end in preference to the solder type. Better yet, buy one of each and after fitting the solder type one yo will know why I think the ones with pre-fitted terminals are the bee's knees.
If it isn't your brushes, on a low useage rate bike the circular contact areas on the rotor can grow verdigris and if can take a few secs to a minute for the brushes to polish that off. A spray of CRC electrical cleaner/lubricant will keep that nonsense at bay.
Next, check your low current terminals by breaking and making them, after that do the same for the high current connectors on the alternator and diode board.
If you have not cured the problem at this point, then you have a wiring fault int he alternator (most likely the rotor cause it is the bit that moves) or a developing fault in the low power circuit of the diode board.
Hint. For diode boards and rotors it is probably cheaper to swap out than it is to pay an auto electrician to find an intermittent fault. When the fault disappears the thing you changed last is the culprit, store it in a safe place for a month and then bin it if the fault has not reappeared.
