Took me another 3 hours before I figured it out. 2 of which to cool my head. :headbang:
It has something to do with the UV mapping ... wanne know?

Well, when u create a polyplane, it still has "Create UVs" mapping options of its creation. Once history is deleted, the options are no more. In the options u can set normalization (default is "Normalize and preserve aspect ratio"). Once turned off, the image is displayed correctly on the polyplane.

bendingiscool: Yep, recreating a new UVset would have worked too. But that's not how I remembered solving it last time. So it will bug me until I get it how I want it. I have this obsessive compulsion when it comes to this. :p