I did some testing. I created a post with a subject but without a body, and vice versa. That generated an error "All of the required fields are not filled in." I clicked on "Return to the previous page" and added the missing content. Then I clicked on Post New Thread, and I got the attached error popup.
The same error occurs if I use the back button instead of clicking on "Return to the previous page"
This seems (to me) to be unexpected behavior. You should be able to post after returning to the previous page and completing the post.
If, after returning to the previous page, I reload the page (which wipes out any remaining content) and then enter a subject and body, I'm able to post properly.
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