I keep getting this seatbelt/airbag light error
 

The error is: 80 Versorgungsspannung

I suppose the translation mean is Supply Voltage; but what does it mean? Is my sensor bad?

What I notice is that if there isn't someone in the passenger seat the seatbelt light goes on and off. And if I lock he seatbelt the light goes away.

Most likely a sensor. It could be passenger seat occupant sensor. If you have BMW Scanner 1.4, try disabling that sensor and if no fault found after that then that is your problem. Also, there is a recall on passenger seat occupant sensor pad for late 2005 to 2006 E53 models. You can call BMW and check if you are part of the recall.

Agreed, might be the recall. I haven't looked for a code, but that's the idiot light symptom on the overhead mine throws [haven't yet taken it in to the dealer to replace the passenger seat pad ......]

Im going to look into that recall. hopefully that is the problem.