The easiest fix is the following. Please note that I'm using Firestorm 4.7.7. The steps/menu items might be a little different on other viewers.
Enable to Developer menu. Preferences > Advanced > Show Developer Menu
Make sure you have your current shape saved somewhere.
Developer > Avatar > Character Tests > select either Test Male or Test Female
After that is done, switch back to your old shape & clothes. Saving outfits should now work for you again.