If the "Roller error" is triggered very often on your machine, even without having yarn tangled on the roller bars, it could be that the ambient light of your space somehow triggers the roller sensor. This can be fixed by adjusting its sensitivity with the two potentiometers circled below:
When the error is triggered, use a small screwdriver and slowly rotate those screws (the potentiometers of the sensors) until you click and the error goes away. There are two screws, one for each sensor (front and back roller). The screw in the front goes for the sensors in the back, and the screw in the back goes for the sensor in the front (apologies about that; we need to ask the sensor manufacturer to flip this logic…). Do this slowly, one screw at a time.
Rotate CW: more sensitivity.
Rotate CCW: less sensitivity.
You want the roller to be as sensitive as possible, but without giving you false positives, so it can detect if a bit of yarn goes the wrong way.