I've just got this from FreeForums.org
I keep getting logged off my forum
Symptoms: After logging into the forum and clicking any link you are logged out and no longer have access to the forums or the UCP. You may still see yourself as 'online' in the board stats but are showing as logged out. If you normally have ACP access when you try and authenticate for the ACP it tells you that you do not have access to authenticate. If you normally have MCP access you are being told you do not have access.
Cause: The usual cause of this is a corrupted or bad board cookie in your browser. You may still see yourself in the stats as logged in but the stats refresh differently and can actually show someone online up to 5 minutes by default (or even longer depending on your settings for this option) after they have logged out.
Resolution: The quickest and best way to get a fresh cookie is to log in and click the "delete board cookie" link on the index page. If you are not able to see that link then you are being logged off immediately as you can only see that link if logged in. Clicking the link will remove the board cookie that your board has placed in your browser and then log you out. Logging back in should get you a new cookie and the log out problem will be resolved.
Occasionally, the delete board cookie link will not work correctly due to your online status or you may not be able to see the link at all. In this case you need go into your browser internet options and delete cookies (this will delete all cookies which is why using the link on the index page is better as it will only delete that one cookie). You may also want to delete your temp files and delete the browser cache at the same time. Close your browser completely and then load it back up and log onto the site.
In some stubborn cases you may have to do this twice or if using the link did not work the first time, try again and this time delete the browser temp files and browser cache before logging back into your forum.
If you are still not able to stay logged in you would need to first determine if your browser is even allowing you to accept a cookie, or is blocking changes to the cookie and therefore corrupting it. In your browser options you want to allow 1st party cookies.
If you or another admin have ACP access they should see if somehow the server side settings for cookie information has been turned on, if so you need to change it to disable.
If you are still not able to resolve this issue you need to try another browser, that has never been to your site before or another computer and see if that resolves the issue.
DW11, try deleting all board cookies. I'm sure there's an option to do that at the bottom of the Index page.