This means you have JS error or you have some plugin which optimized/minified TinyMCE editor and break it. The TinyMCE editor is crashed. Please make sure you don't have cache plugins activated for loggedin users on forum page and disable optimizer/minifier/compresser plugins during the test. And please delete all caches before checking.
I'd also recommend disable recent installed plugins one by one and test. Some of those generates JS error on front-end and affects TinyMCE editor JS functions.