Try the following. It sounds strange, but it might work:

Pull your entire downloads folder out and put it somewhere safe (your desktop, for example). Start your game without no cc loaded, then shut down and put your downloads folder back in. Restart your game and, when asked, enable custom content (it will automatically be disabled). Close the game and restart.

That might do the trick. At least I hope it does.

Also, try deleting your groups.cache file

Does your Download folder have a capital D? I'm sorry I am trying to think of anything I can. We are gonna figure this out.

You have to enabled the custom content by click on the one of the options ( i'm not sure which one i think its the one with the picture of the gears) and look for enable custom content and then turn it on...i hope this helps