I find that when this happens to me I highlight what I've written and in the pop-up menu click 'cut'. Then I come right out of the forum I was trying to post to, to a totally different forum. Then I go back, click and paste my original attempted thread and once again click on 'Post'. Long winded I know but it works for me.
LG G6 review: Hands-on with LG’s bold, big-screen shot at perfection