Thank you for all the reply so far.

The door imbalance / rail track is not a problem as I have
dis-connected the door. This allows me to isolate the opener problem or
door problem. Actually, I try to lift the door manually (without the
opener) with single finger and it is confirm in perfect conditon.

The chain tension may not be a problem as I also dis-engaged the chain
from the driving gear and the opener still partially open.

Winston