Thanks for your response chrishajer. You made me realize I had put the quicktags-4-bbpress folder in /my-plugins/ instead of putting the files themselves in /my-plugins/. (Although in my defense, I was just following the instructions of the readme included with the plugin). Just moving them directly to /my-plugins/ didn’t fix anything.
Since BBpress plugin DB seems to be down I decided to download the plugin from 40annibuttati.it and uploading those files fixed my problem. So maybe bbpress DB has a different (wrong?) file? Anyway, this is what worked for me: