According to Brother,
solid light on error means no paper in the tray, or that the printer wants paper in the manual feed tray. If someone has submitted a job that, for some reason, is using a non-standard paper size, it's possible that this keeps getting sent to the printer and the printer then displays the error light to ask for you to put paper in the manual feed.
It is also possible that the sensor to detect out of paper is bad. Usually out-of-paper sensors are some sort of a mechanical assembly where a plastic swing arm rests on the paper one side, and on the other it breaks contact with an optical slot sensor. Tiny bits of paper, or, over time, even accumulations of toner or paper dust in that optical slot sensor can gum up the works. Unfortunately, finding that sensor can be a pain - they are generally not accessible or even often even visible.
