Mac OS has incorrectly set the permissions for some files.
When you upgrade a Mac OS (as opposed to installing the new version fresh), Mac OS will _often_ set permissions of folders and files incorrectly.
You'll see this problem in programs like After Effects in the form of error messages saying that a file can't be written, overwritten, or renamed.
To fix this in the case of After Effects, go to the preferences folder and set the _entire_ preferences folder ( /Library/Preferences/Adobe/After Effects/12.0/ ) and all of its contents to be writable, not read-only.
Thanks for your help; it was for the 12.1 version (I had to do the same thing for 12.0). In any case, your solution fixed it. Thanks!
How do I find the Preferences folder on my system?
Follow the file paths mentioned in this thread.