If you open an Office Document, you might occassionally find your self with an error that tells you that your “object library is invalid” , “excel has had a catastrophic failure” or you have an “invalid reference”. Once you have this, you cannot save the file, and depending on the version, Excel or some other office program will just exit. Even opening it in safe mode doesn’t help.
Almost certainly you will have just have had some kind of windows or office update, that will have changed one the libraries. When you access some files, Windows makes a cached copy of it (called an .exd file).
Amazingly, the Windows update process does not clean these .exd files, so that means that the cached file may be out of synch with the new version of the library – eventually you end up with the error described.
It’s very easy to fix – just delete all the .exd files. For details, see this post from 2009 by Lessan Vaezi. Incredibly, the same old thing continues to happen today.