For some reason, these controllers calculate speed differently when under power and when coasting, There is always a speed signal from the wheel for rpm, and they use the wheel size to convert that into mph when coasting, However, under power, they use the motor speed and convert that. To do that, they need the number of magnets in the motor and the internal gearing. For your S866, that's P07, and 86 is typical for a geared fat bike motor. If you put in 0, it might make the conversion go nuts,
Meanwhile, Error 10 is a communication error for your display. A random error 10 might mean the data bits are jumping around between the display and controller and cannot be read cleanly.
Try setting a non-zero P07 and maybe unplug/plug the display connector in case it is loose. Probably a new S866 will fix it. Maybe it cannot keep up with the calculations needed to figurre out speed.