Light flashing after opening

Hi I have a liftmaster 2000 remote garage door opener.

After opening the door, during the brief period when the light should be on, it flashes on and off rapidly and I can hear the relay clicking.
The unit will not respond to the remote control during this time.

After a while the light stops flashing and it is possible to close the door.

At first I thought it was a faulty blub but the replacement does the same thing. I have reprogrammed the remotes and reset the unit but still have this problem.

thanks
Steve