Error on post
Error on post # #kbinEarthError on post
Error on post # #kbinEarthYep, your text is too long. Below the textarea (left corner) is a counter which shows you how much chars you used and how much the maximum is. Here the max is 5000.
I'm surprised it let you submit the post and not show an error with an proper message.
I can see why it troubles you. But on the other hands admins and devs need to set limits to prevent bad actors from using up the server resources with just a few requests.
I'm sure that there are other Fediverse platforms which allow much larger text, so these might be something for you (or your host your own as your said).
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.