Yes, just after filling the Flux Survey and mentioning I did not have a problem with Error #121, my Flux decided to prove me wrong!
I used my machine daily, since day one 6 month ago, without any #121 until recently. The errors started to appear gradually, one error every 2-3 prints, then progressed fairly rapidly. Recently to many errors started to occur in the same print, which I resolved, like many others, by turning off the filament detection in the Expert mode.
@proclaim, I checked the filament switch visually, it looks OK, no visible wear and tear. I am suspecting the error is caused by switch contact oxidation with time and ambient humidity. I am sure these cheap switches don't use gold contacts and it does not take much oxidation to cause false switch readings. I am not sure how the Flux software is detecting the switch response but maybe a simple "timing loop" could prevent these false readings. If the software waits 2-3 seconds for more switch readings before putting out the error message, it would resolve most of the #121, since when I hit continue when the error message is up, the printing continues until the next #121.