After Effects just recently stopped accepting copied text from Microsoft Word (and many other applications incl Photoshop).
Although it does allow me to copy into the AE GUI search bars, but still not from there into my composition text.
I have tried a system restore to before the problem developed, however it did not work. I also found this (http://shisko.blogspot.co.nz/2008/01/copy-paste-bug-in-after-effects-fix.html), but the patches are for XP and Vista.
I'm on Windows 7, running AE version 126.96.36.1998, let me know if you have a workaround! This is very frustrating!
Begin by installing the most recent updates and restarting your computer.
You have an external tool that is messing up the clipboard most likely. Check anything from Skype to screenshot utilities...