Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
Could you tell me exactly what the 403 page says and what URL is in the address bar when it appears? Does the 403 page carry InfinityFree’s branding, or is it a plain “Forbidden” page? That would show whether it’s our hosting refusing the request or something in phpBB or your .htaccess.
I’d also like to know whether it happens with every message, or only some. Try a one-word post, then a longer one, and tell me if both fail.
I found two articles that could be related, but nothing yet that points at your case.
The first covers the browser security check. It says the check only stops tools and apps that can’t run JavaScript or keep cookies, and that real browsers work normally.
The second lists URL keywords we block automatically, such as chat and includes. Your posting URL doesn’t contain either, but the exact URL in the error would show whether another one does.