I haven't seen that before
A quick
Google search seems to suggest there are permissions issues on your disk (Disk Utility, fix permissions and might as well check the disk too) or the library (or one that it uses) is corrupted.
The former is an easy thing to do. The later would probably be localized with the CarFrontEndAPI framework (since you'd have many more issues if Cocoa, Foundation, or AppKit were broken
). I would suggest deleting the copy you made, then do a Clean All on both projects and try again.
Another test would be to Clean All both projects, install CFE using the installer, then change the CarFrontEnd project to use the installed CarFrontEndAPI (/Library/Frameworks) version and see if it will build and run then.
If you still have issues, take a look at the HOWTO thread for building plugins and see if any of the info about build issues might apply to your config.
Hope one of those ideas helps.
-dave