Sometimes people get the error
orNo post mode specified
When they try to preview or post a message on this Message Board.Not Acceptable
An appropriate representation of the requested resource /forum/posting.php could not be found on this server.
Apache/2.0.51 (Fedora) Server at http://www.scramble.nl" onclick="window.open(this.href);return false; Port 80
This has to do with some security settings our hosting provider has set on the web server this Message Board runs on.
This error seems to occur when you try to post something which includes the string "to:" in the message (so the letters TO followed by a colon). As this string is part of a lot of scripting languages it could indicate an attempt to hack the site.
When you get this error, try to rephrase your message, for instance by putting a space between TO and the colon, or replacing the colon by a semicolon.
Note though, this string as part of more regular phrases, like photo: !!
Hope this helps.
(thanks Chris for helping to solve this one!)