This afternoon, when trying to import one of my FS9 planes into FSX, I had this problem that the screen was compressed in height, and all the bottom part of the screen (let's say 1/5 of the screen) was a black.
I could remember reading something about this in this forum, but after making some unsuccesfull search and reading almost 30 pages of topics, I gave up.
Then suddently I remembered that it was something with the 2D panel. So I went into the panel.cfg of this plane and erased all the [Window_xx] sections, which correspond to the misc 2D panels (main panels + additionnal panels, electrica, map, etc...)
It worked.

I don't know (and I'm too lazy to test right now) if deleting only the part corresponding to the 2D main panel would be enough, and I don't know either if there is any alternative to the deletion.
But for the people out there who doesn't care at all for the 2D panels, it could be a good quick work-around to make a plane work correctly in FSX, what do you think ?