I just double checked in the manual. Engine capacity is 6.5 L. So I have significantly overfilled. I'm glad that I haven't driven to work like that.
So i suppose it must be the sensor - as I can't think of any other explanations.
I've had the car for 3 years and had my local mechanic change the oil and no issues. I finally decided to start doing it on my own (not to save money but for self satisfaction) and something goes off with the sensor.
Unless anyone else thinks of any other culprit..