A prior cookie containing a session ID that is no longer valid seems to be what causes trouble like this, even though that's an expected scenario.
"Clearing the cache" was mentioned, but cookies are cleared from a separate area. You can see and delete the cookies by clicking beside the URL in the Chrome address bar, and entering the cookie management from there. In this screen shot, I've circled in red the places you would be clicking.
You could also instead try using the "Delete Cookies" link at the bottom of your phpBB site's index page, in case that's easier.