The first thing to do in a situation like this is to run your game with an empty downloads folder just to double check that there is nothing in there that you don't know about that could be causing the problem.
You don't have to delete any of your CC (not yet anyway), just move your downloads folder to your desktop.
Run the game and see if Sims can interect with each other. Don't save when you've finished testing though. If they can interact then there is something in your downloads causing the problem. If they still can't then its probably a faulty install.
If it was something in your downloads folder then you'll have to slowly put the files back in and keep testing the game until the problem comes back. When it does come back just check which the last files were that you put back and it should be one of those causing the problem.
This thread over at MTS2 should explain in more detail.
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