As far as I know this message is peculiar to ConferenceRoom (CR).
CR V2.0 has a mode +M which once represented the Malaysian client class, but due to CR's implementation of some IRCX properties the +M now blocks private chats. If you are on Webnet which use the CR software then this is likely to be why you cannot send a message. Just type /umode -M (Make sure you use the BIG M not lowercase as modes are case-sensitive.)
