Is this happening with a specific project? Can you try with a different project. Although it sounds like a corrupted footage file. Please try it with a new project with no footage to test for this.
Sounds to me like perhaps you might have a corrupted prefs file or even a corrupted After Effects installation. I would try deleting the prefs first (hold down Shift-Alt-Cmd when starting AE) to see if that fixes things. If not then I would try re-installing AE.
I got the same error message using CS6 when switching between German and English language in Creative Cloud. Funny thing: I had CS6 installed and it worked fine using the GUI but Creative Cloud showed AE CS6 as not being installed. Resetting preferences etc. didn't help. Reinstalling CS6 via CC helped. Didn't have to reinstall all my scripts and plugins since they are not deleted during the process.
I hope this helps