I have the same problem - the HTML editor is not working for 1 language, but is working for 2 ( I have a multi-language site) and I could not figure out why it is not working for 1 language.
Finaly I gave up searching and I compiled all texts in one html area and copied them as plain text to the language where the HTML was not working.
This is just to let you know that maybe you should consider a clean install.