I get it sometimes. I refresh the page and it's okay after that.
This will also happen if you cut and paste from another site and have any non "ASCII" (Text Characters) or what is referred to as Unicode characters or other language characters.
You can clean up the text by first pasting it into notepad and deleting anything that is not text and then cut and paste it to the forum. This will make it pure ASCII and the message will post fine.
There are other times that the board does not have enough connections to the SQL database and you will get this error when reading the forum. If that happens, you should wait a minute or two and hit the refresh button on the browser.