Okay I've had this bug for several weeks now and I ask myself if it only happens to me: Sometimes I need to login twice to be actually logged in. It's not quite a problem, but after logging in the first time, phpBB still recognizes that somehow and resets the "last time logged in" timestamp, so I won't see new post markings in the index.
I'm using Chromium. (on two computers, also happens with Firefox)