I have just completed the quest "the man who waits". When leaving the sanitarium an NPC approaches and the quest with Lady Webb starts. In front of her house a "old robed man" appears and speaks to me about a mission and that I have gone astray. This seems to be totally out of the storyline, especially since my possible answers are few or only one, concerning something about that I excuse by his excellence etc. Is this a bug or does this encounter belong to a quest I might have started without knowing?

You've seen him before. This is part of the main quest.

I remember him from the main quest. However, is this dialogue at this point at this place right? Because it sounds as if I had a mission which I neglected. And this is the part I cannot remember.

Just keep going. It will (hopefully) all make sense before the end of the game.

It won't get out of my mind, I have to ask again: Since I cannot imagine that the answer-options in this encounter are forcing you to say things you as the main character have not done and the behavior is so unlike the character played till here, I assume that it is a memory from someone else (since here, like when you reach out for a soul, the surroundings get purple) and, unlike the other reaching-outs, you can choose answers concerning past things, i.e. the answers and behaviors in this scene are not the ones from the main character (or at least not in this incarnation) but the memory from someone else with the difference that this time I can choose and so to say form the memory. Right?

Yes, the memory is of something else. No, they are not from this incantation. Yes, you can choose what to say.

 

Almost like a choose your own adventure story...or a role-playing game.

Correct. If you want more info,

 

It's a previous life lived by your character's soul. Your answers only let you somewhat define what kind of person you were in that life.

 

I must admit, I am positively surprised that my conclusion was correct. :D