While I compose a message in the Rich Text editor, the editor is nice enough to autosave periodically, However, sometimes it displays the error: "Sorry, unable to complete the action you requested.
Usually if I ignore it and keep typing, the problem goes away. Sometimes when I press "Post" I will get an error message as well. In that case, I usually have to restore an autosave version.
This happens to me about 30-50% of the time that I respond to a post, and even when I originate a post (like now). Possibly relevant: I take a long time to compose responses, so you might try to reproduce it by slowly typing/revising a message over 5-10 minutes.