I’m using CS6 and just ran into this problem. Not only could I not drag a specific file into AE, but I couldn’t even use Ctrl-I or File>Import without getting the error.
Adobe has a help page for previous versions that mentions replacing some files, but didn’t work.
What Walter states is true and my problem was solved when I found a folder name on an external drive that I was accessing suddenly had an additional unusual character. Once I removed the character everything was fine.
Thanks!