stik,
I found out the answer a little while back, but didn't know you were still having these problems.
Basically, it's because phpBB changed the way it does session tracking in the latest phpBB releases, for increased security. Unfortunately, AOL makes it
very difficult to do session tracking, and it confuses the heck out of phpBB sometimes.
Here's a technical explanation (unfortunately, phpbb.com is down right now, but check a little later). We're looking into seeing if we can develop a workaround.
--janak