"Confirm Action" Problem

I am using punbb 1.3.1 running on Mac OS X server version 10.4.11. When I first installed it, everything seemed fine. But now, whenever I try to log in, I get a message saying "Please confirm or cancel your last action". This happens even when I clicked the login link less than 15 seconds ago, so there is no way "that some time passed between when you first entered the page and when you submitted a form or clicked a link", as the message suggests. If I click confirm twice (the first time I click it the message just comes up again) it does let me log in, but obviously this is not acceptable behavior for a production forum. I get the same behavior in the Administration section - I click Administration, go to a page with an action link (such as installing a hotfix) and immediately click the link, only to be greeted with this message. How can I fix this? Thanks.

Israel