This staircase suddenly became unusable. After a rather long search for the cause, I discovered that the garden gate was actually responsible. I moved it only a tiny bit—as you can see in the pictures—and the staircase immediately worked again.
Now I’m genuinely curious: Why can a garden gate in this location block a staircase so far away? And why can it be placed there without any warning if it causes this kind of routing issue? You really have to figure that one out first. 