In case you get the "Motor Sensor Error"...

[pongo1366]

The usual advice is to check that the rear wheel is properly tightened, and in most cases it already is.

What fixed the issue for me was removing the rear wheel and checking the sensor. It turned out to be covered in brake pad dust and debris, probably because the rear brake sits directly above it.
Pads are still Amflow's original.

After cleaning the sensor and removing the residue, the problem disappeared completely. It may be worth checking this before replacing any parts.

 

[Yotaismygame]

Ill have to check. Mine at 100 miles has flashed this warning twice the last time out . Always on power up. Switch from off to the tune i want, one or two pedals it would flash on and then turn off in a couple seconds. So i would have to again toggle from off to the tune and away I went no more issues. Was not my rear axle.

 

[pongo1366]

Ill have to check. Mine at 100 miles has flashed this warning twice the last time out . Always on power up. Switch from off to the tune i want, one or two pedals it would flash on and then turn off in a couple seconds. So i would have to again toggle from off to the tune and away I went no more issues. Was not my rear axle.

Same as me.
In my case, the rubber grommet that holds the cable to the frame came loose, allowing the cable to sag. It then rubbed against the brake disc, slightly damaging the insulation. While waiting to replace it, I wrapped the damaged section with electrical tape, but the problem only disappeared permanently after I cleaned the sensor.