Recently I've been getting a pop-up with this error when trying to preview or post a comment, whether just a normal one or replying/quoting somebody else (as in this case, I posted to the thread in the link):
I've also noticed that I can get around it by making it shorter but there doesn't seem to be a consistent pattern to the length (i.e. not caused by a hard character limit), at the same time, I was able to post to the thread linked above when I tried again while making this thread, without making any changes (I also noticed that I got one of those "checking your browser" screens when making this thread, which I've never seen before except when looking at my/somebody else's profile).
What it sounds like is happening is that your post is triggering Cloudflare's security (which looks at the contents and would stop posts if it thinks it's bad). It's... not a great system, and is just trial and error to find what triggers it.
The weird thing is that the exact same post went through without a problem after I'd made this thread, which as noted brought up that security screen (when it comes up when looking at a profile it won't reappear for a while, I'm guessing it attempts to show it but it doesn't work properly when previewing/posting comments); in another post removing various random sentences/words fixed it, making me think it was related to the length but much shorter posts have had the same issue.