After a few months - the same problem came back. I was itching to find out what exactly was causing this. So I removed the tail light assembly. It was not dirt causing the problem. The culprit is "arc"ing between the bulb holder and the steel frame. Over time this poor connection created a "pit" of non-conducting material (kinda like a carbon track). I pointed it out in the first picture.I just cleaned the black bulb holder with a clean towel and the socket especially. Dirt accumulates and creates resistance which causes the error.