What I found on another site (you probably saw this already):
Basically this feature protects against hacker attacks by giving every active user a Session ID, part of which is derived from your current IP address. If your Internet account uses dynamic IP addresses and a timeout so that your machine goes offline and then online again while you are composing posts your IP address will change and your Session ID will become invalid. phpBB will then assume that you're a hacker and not allow you to post.
Users of AOL worldwide and T-Online in Germany will be particularly strongly affected by this problem, as will all other users with providers who change the entire IP address every time you log on.
If you're interested in the technical details see this message from the phpBB staff:
http://www.phpbb.com/phpBB/viewtopic.php?t=69493
Workaround:
If you experience this problem while trying to post a message or reply do this:
1:
Immediately click on BACK in your browser. This will display your editing screen with the text you just entered.
2:
Copy all the text you wrote to the clipboard, then return to the forum and select New Message or Reply as appropriate for what you were trying to do.
3:
Paste the text you just copied into the editing window and immediately click on SUBMIT, while your current session is still valid.
This is really lame. I'll see if there's some way to prevent that. I'm not afraid of hackers taking my board on.