Limited rotation in 3D

I have drawn an end view of a steel duct in the XY plane which I have extruded along Z. I wanted to look at it in 3D from an approximately isometric viewpoint. So I rotated the view using Shift-click, but my rotation options seem to be limited. It seems as if there is a "lock" on any horizontal rotation and I can only rotate vertically. I think it's related to the pivot, but I'm not sure.

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Clik here to view.Image may be NSFW.
Clik here to view.Image may be NSFW.
Clik here to view.

Image may be NSFW.
Clik here to view.