If I trip the sensor by driving the car outside, the light goes on. The remote won't work. If I then go in the garage and push the button to turn on the light and then leave the garage without tripping the sensor, the remote functions normally.
This is a Chamberlain Lift Master using a Clicker remote. The problem started recently after 15 years of faultless service. The EMF idea suggested in comments sounded great, but didn't pan out. I tried a new battery, but no luck there. In short, when the door opener light is on, the remote won't work. When it's off, the remote works fine.