XCode 4 Archiving Error: one or more PCH files were found, but they were invalid
I had to read this a few times to get what was going on. The original poster answered his own question. To fix:
Clean the Build Folder by holding down ALT and selecting Product from the menu. The "Clean" entry will now have changed to "Clean Build Folder" which will wipe the bad PCH duplicates and compile without errors.
I was experiencing another problem where this solution helped. After updating an external library I rely on, I got this message:
fatal error: file '/path/to/SomeHeaderFile.h' has been modified since the precompiled header was builtUsing a "normal" clean already helped getting rid of it for Debug builds. But when I wanted to use Archive in order to create a new build for the AppStore, the error message remained.
Using that "Clean Build Folder" now finally solved the problem for me!