Hi, I am having the exact same error, and problem. Which ticket did you have and what did they send you. I have this at the nearly end of the print, and I also disassembled the whole thing, there was no filament viewable. how does this sensor look like?
For me it is on the left extruder.
I had this once with pla which was wrapped around the extruder wheel, which I cleared, then it worked, until now, it does not at all. The thing is clear and absent of filament. however, can this be a hairlike bit of filament? Where is the sensor?