I had my phpBB2 setting pretty much as you mentioned. I had the expiration set to 10 days instead of 7 but changed it to 7 anyway. Doubt that could be an issue as the problem happens in less than a day. But at least we are on the same page to be sure.
I PM'd you some cookies values after a re-occurance this morning of the problem. I sent before and after re-log in values. Maybe these will help shed some light?
I re-uploaded the stock phpbb2.php file. That was the only change we made.
Let me know if there are other things I could do to help you.
Last edited by Venturous; December 26th, 2005 at 12:44 PM.