I had a problem like this on my old computer after one of the patches and was able to fix it as follows: (Note, you have to do this every time you want to export a sim to the exchange.)
You can cut your DCCache folder (located here: C:\Users\Marnie\Documents\Electronic Arts\The Sims 3) and paste it to your desktop. Then you should be able to sign into the game, go to CAS, export your sim to the launcher and then upload it to the Exchange. Then take the same DCCache folder that you moved to your Desktop and put it back where it was, just write over the new one the game created. I've tried this with no problems and you don't have to reinstall any files. If you don't put this folder back, you have to reinstall all your sims3pack files via the launcher
I suspect this error is happening because there's a corrupt Sims3pack file in the Downloads, but until you locate it, this is a work around.