Sometimes this may happen when the Hosts edit the part of the forums that you were interacting with.
Sometimes this may happen for no apparent reason.
When it happens to me, I copy my intended reply to a text document (as backup independent of the browser), then reload the browser page. Then check if you are still signed in, by looking at your profile image in the upper right corner. If you got signed out, then that was the issue, and solve it by signing in again. If you are still signed in, then see if the web page remembers your not-yet-send reply, and confirm that you would like to resume the message from restoring this content. If the web page doesn’t offer to resume your reply, or still refuses to save the reply, then force-quit and a relaunch the browser, sign in again, reload the thread from History, then use the copied text from the first step.
Good luck!