As flobbeh said, it should usually just be to click send again a few times.
But without any luck doing so, it's most likely due to you being logged into the site and the forum at once. Suggestion is really to log out from the site in the box next to the banner, and it should log you out from the forum as well. Once that is done go back to the forum index, and log in to the forum only and do not use the login box next to the banner.
Try to submit your message again a few times and it should work.
This error message you get is usually because you took too long to write the message and you need to reload the page for it to work again, but if this doesn't work it could be due to you being signed into both the site and the forum at once. If you did try to log off the site and just log on the forum without any luck, Deleting cookies as mentioned above is the first thing you'd want to try.