The most likely suspect is the optical sensors mounted just above the floor level at the door.
Test them by putting something in front of the 'eye' to see if the light shuts off. If both lights shut off when blocked, the problem is with the opener unit itself. If one of the lights doesn't shut off, it is either a faulty sensor or a misalignment of the sensor. Look closely at the sensor lights. If they appear to be green all the time, but in actuality flickers almost inperceptibly occasionally, it is likely a misalignment of one of the sensors. Adjust and test.