If I try to export a xmodel from CoD5 maya gives me this error:

# Error: TypeError: cannot concatenate 'str' and 'tuple' objects # 

I did not try exporting from CoD4 because I dont have the game.
I tried using the "old" CoD Maya ...

 

When I had the same problem that you, I had forgot to select "tag_view", and sometimes I fix somethings
randomly selecting"tag_view" when exporting the xanims, you can try something like that, I don't really know what is causing this 

What joints did you selected when exporting the xmodel and the xanim?

Have you tried something like not having zones adjacent to each other?

I'm only used lambert

You need to apply the material in maya like "mtl_mycustomweapon" and convert the material with the same name in Asset Manager