I think a screenshot can explain it better than words. The only affected viewport is the first one (i.e. the first context) and changing its perspective (e.g. to top or front) doesn't have any effect. Likewise the GL versions 1.1 to 3.3 make no difference.
However when, instead of launching Houdini directly, it start it by clicking a scene file, the issue is gone!
I'm on Linux with current proprietary fglrx drivers. Any ideas?
edit: I'm sorry, I just realized I probably should have posted this in the Technical Discussion section.