Get rid off "false" Interrupt Signal when Motor starts

you mean like this (image added)

if you say 10nF is plenty, do you mean 10nF is better or is 100nF better (more secure), but 10 would maybe also be enough?

unfortunately, I can't change anything directly at the sensor or at the motor, the machine was designed by someone else

PS. maybe I'm lucky and you've also got an answer to my second question