There seems to be an issue with posting content from Safari from a page that's been open for a while.
We upgraded our webserver (nginx) over the weekend, and this seems to be a bug between that version of nginx and Safari 9.1.
We'll look into this in more detail tomorrow.
In the meantime, either post again, or use another browser for now.
I (Calvin) personally use Safari 9.1 as my primary browser, so I'm affected by this, too.
Details here: https://forum.nginx.org/read.php?2,266525,266525#msg-266525