Thanks for all the replies. The problem went away when I let the door
open fully until it hit the upper limit. My guess is there must be a
force switch which, when activated by a door obstruction, can only be
reset by fully opening the door. However, due to other objects hanging
from the ceiling, I usually never let the door open fully. I shut off
the power before its opens too much. So I disconnected the door from
the opener arm, and let the arm retreact fully. That solved the
problem.