Can no longer edit ANY swatch palettes after saving a custom swatch file

Using Adobe Illustrator CS6 on Mac OS 10.7.5:

I saved a custom swatch palette for one of my clients as a .ai file, and imported it into several of their files. However, now I can no longer edit ANY of my swatch palettes. That is, I cannot create new swatches or delete existing swatches. The icons aren't even there. 

It gets worse. Even if I open the default swatches, same issue. Worse still, it's happening with ALL Illustrator files, so even files that I've never imported the custom swatches into will not allow me to edit default swatch palettes. 

How do I fix this?