I would say, there's maybe an issue tho, I tried posting the following answer all evening:
https://pastebin.com/mHHuLk16
I cleared browser cached data, ipflushdns, tried multiple browser (even installed another one after asking help on the discord, thanks to x.iso btw

), checked system and browser updates, restarted my machine, etc, couldn't get it posted.
But if the message is:
https://pastebin.com/hynEfUD8
It passes. (both sending and preview) It was, in my case, related to that specific thread:
viewtopic.php?f=3&t=6892
I could pinpoint what part of what I'm sending is causing issue tho
"So, is there a "resource-less"/minimal/core packaging of hh5 somewhere? If not, is it something I might have the joy to see one day?"
If that part is not present, I can either preview or post, if that part is there, I have the following error:
Not Acceptable
An appropriate representation of the requested resource could not be found on this server.
Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.
I thought it was maybe an auto-quote system thing or something as that quote is a copy/paste of my original message, but modifying part of it don't solve the issue.
Also, if the quoted sentence is posted/previewed alone, backend can process it.
Maybe a " ' / escape issue? Don't know, but the issue seems real, if related, and not client specific at all, really looks like a server side validation/formatting/etc issue :/ That would fit the OP issue as, using another browser, to post different content, if it's the content that's responsible for the issue, will not have the issue happen, but that might be a totally unrelated issue and just a bad day in paradise.
It's both on posting and preview. .
If you want more inputs from the client, hit me and I'll help as much as I can.
Best regards
Emmanuel