Thanks. I have replaced the qa-plugin/wysiwyg-editor folder with the one from the dev version and now I can see the textarea without any further modification in the code. Anyway, I still do not see the editor toolbar.
I'm not comfortable using a dev version, that's why I didn't replace every folder of qa, plus I don't know if that will solve the "no toolbar" issue.
Edit: I've found that the textarea is shown only if you reload the page from the browser. On the first page load, the textarea is not shown. (you cannot write an answer or a comment)