Wow. I opend your file and i have no clue, sorry.
Do you have the mesh in a good state sometimes? I you have you could try rescueing it by copying the mesh: All Shapekeys must be deleted (thats the problem: you must animate everything again). Select all vertices and press Shift+D and the right mouse button as you dont want to move it around. Then seperate the vertices to a new mesh by pressing P and select "Selected" from the popup.
Then you have hopefully your mesh as a new object and can work with it again.
Do you remember if it was a problem by the importer or only a corrupted file by a bad save or something? Means, have you edited the mesh so far?
If it is the last case i can only advice you to try importing the basis g3d again and edit it to see if you can find out after which step the error acours(?).
Hope we find a solution somedays
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