I too have had that error message when trying to post in recent weeks.
I suspect the forum data cache is getting very full or somesuch.
I suspect there is something in the posts that we make that triggers the rejection. Not sure what it is though - might be clicking one of the emojis at the bottom of the toolbar but why that should be, I know not. If I do a larger post of a few sentences, like this one, I copy it before pressing the send button and if I get the error message, I can easily post it in a fresh post. That almost never fails to work.
John
