"Your post has been changed because invalid HTML was found in the message body. The invalid HTML has been removed. Please review the message and submit the message when you are satisfied."
Even if I did ctrl-A (select all), and just wrote some text, I had the issue. And even after I clicked 'discard message' and trying to write the message, I got that issue.
I see no option to reply to that thread so i'm posting a new thread here (And isuggest you enable replies to old threads 'cos people look them up)
Solution for me was see it says "Rich Text" or "Preview"..
Clicking Preview (before clicking "Post"), worked for me
BTW, this time it let me use 'rich text', but in an old thread I had quoted somebody and maybe that was part of the problem.. Or i'd copy/pasted from within a quote, or tried to snip parts before and after the relevant part of what I was quoting of them.