I did a reset (Acad was already updated with the latest update). But... that didn't resolve my problem. I started another file, based on another template and this one DID allow me to add text styles. The file I was working with still doesn't allow me to add text styles when I click on the "+" in the text style dialog box. Maybe it's really file/template related? Can someone from Autodesk confirm?

Anyway, my problem is resolved. Thanks for helping.