Are you using the Res Fix or some other mod/edit to change the resolution of the game when you play it? Because that can cause issues with Mission Editor.
If you're using the Res Fix, rename the d3d9.dll file in your game folder to something else before opening ME, that will temporarily disable the resolution change. Just remember to rename it back to d3d9.dll before loading the game again.
If you're using some other mod/edit to change the game's resolution, you can try temporarily disabling or removing it and see if that does the trick.
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