Pretty sure error code 400 in this case means there was something wrong with either the post body, lang or isAdult fields. lang and isAdult should just be set to their defaults so they're unlikely to be the problem.
How long approximately was your post? I'm unsure what the maximum length allowed is but if it was very long that is most likely the issue. It might be worth splitting it and putting part of it into a comment under the post. Hopefully it was saved into the drafts so you don't have to type it out again.
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