And the board itself works (having correct cookie settings which won't log you out every now and then)? Also keep in mind you actually need to request the editor page before submitting it, as it comes with a few hidden parameters which must be sent and change per request (hence parsing that page is mandatory) - but you already wrote you're sending the same form parameters, so that can't be the problem...
Of course: different user agents can be treated differently. You can check this by just using the same one as the internet browser you're using (along with the other headers it is sending).
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Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36