It appears as the center section is sinking like you suspected. The green doors appear to be plum, but the jamb is out of square. As a carpenter this drives me mad looking at it. However, if the foundation is bad it's big $$ to fix.
Do you have a photo of the upper portion of the building?
Sorry to say I don't have a photo of the upper portion of the building. But now when you mention it, I will have to look at it better next time I'm in Köben. But there are more houses and doors like this one in Köben, I will hopefully have time to post one today after work. And the great thing adout that door and house is that there is an even more askew window there and I will post it to in the same post.