...and, due to a total meltdown of my motherboard, I treated myself to a new cpu.

And now, everything fits... It has to do with the combination of the text size you use and the resolution the member has. I can see everything on one screen-width now, and can recreate the problem by changing the text size I display (but it only affects the posts and not the menu bar) or I can recreate it in total by using the magnifier...

So... others who experience the problem can try altering their resolution.

In any case, thanx for looking into it.