When it tells you to open the scrip editor, you should open it, and it should tell you what the problem is.
What you will probably find has happened, is you've got a hole in your mesh that you haven't noticed.
Go to>> Mesh>> Cleanup>> and in the options window make sure "Nonmanifold Geometry" is checked.
Hit apply, and it should highlight the vertices's in the problem area.
You will probably have to manually fix the problem, weather you merge the problem verts, or have to delete them, depends on the problem.
"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
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