No. It's true you can look at the same door and use either phrase, but an open door is a "door which is open", while an opened door is a "door which someone has opened". Passive voice. ;) So the "opened door" tells me someone is around!

an open door = a door that is open an opened door = a door that has been opened The difference is minimal.

Yes, they're the same thing! :)

You must use "an" for "open door" - "an open door" and, I have noted that the sentence "the opened door" is used so rarely.