If you get this error message please click your back button and click submit again, there is no need to copy and paste the message.
The reason for these errors is because of an added security patch in the new version of phpbb. Removal of this makes the forum more vulnerable to hackers so they don't advise deleting it. It has something to do with comparing the users IP's to their session ID's, users using AOL may experience more problems because their IP address is constantly changing, therefore by the time the user has written the message they have a new IP address and then the system is unable to pair the IP and session ID together.
More information about this can be found on the phpbb forum:
http://www.phpbb.com/community/viewtopic.php?t=69493
Hopefully this issue will be ironed out in the next release of phpbb.