Doors are important. When I was growing up, we had screen doors. Much of the year the solid doors were open. The whole house felt open and inviting. There is meaning when a door is open, and when it is closed.
Doors are open for hospitality and locked for safety sometimes in fear.
Passing over thresholds can symbolize significant steps in life. We don't expect or want problems with doors. Obviously, there must be a problem with the door. They were in the Upper Room, that place which had meant so much for them during the Last Supper, that room which had been a refuge during the horror of Jesus' death and the long hours after he was placed in that cave. The Upper Room was hopefully a place of security for them, a place filled with memories.