Hi Steve, I’ve never had this problem, but first of all I would check if you have more than one device connected to the same post (e.g. you have opened the same post on the PC and the phone).
Another thing could be that someone in the forum has edited your post as an administrator while you were editing it (if a user is very helpful and active on the forum, they gain the ability to edit thread titles or move them to different sections).
This happens to me since a few weeks every time when I try to post a new topic. When I ignore the warning and just hit the button again, it completes my post.
However, a couple of times I didn’t had luck and was unable to create new topics, so the only workaround was to copy my whole post, save it as a txt document and try later.