It seems allot of people are getting errors after upgrading their phpbb port to 2.0.15 from any version. Particularly the error below:
phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1054 Unknown column 'session_admin' in 'field list'



INSERT INTO nuke_bbsessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('b7f1c0a7f300d3ca9244450bae4bb57b', '1', '1118225127', '1118225127', '40c14413', '0', '0', '0')

Line : 203
File : sessions.php

You must run upgradedb.php (something allot of people apparently are not doing) from root once the upgrade is complete. This is only if you have upgraded to patched 3.0. Call it by entering http://yoursite.com/upgradedb.php in the address bar of your browser. If you have just upgraded your phpbb port from another version you must USE ALL THE PRIOR VERSIONS to arrive at phpbb 2.0.15.

For example if you are currently at 2.0.11 and you intend on uprgrading to 2.0.15 then you must use 2.0.12, 2.0.13, 2.0.14 AND 2.0.15 the versions in between and run their installers. YOU CAN NOT SKIP ANY OF THE STEPS OR YOU COULD BREAK YOUR phpbb.

Note: If you have installed chatserv patched 3.0 then you SHOULD NOT INSTALL 2.0.15 it's part of the patch and you could break your forums. If you intend on installing chatserv patched 3.0 then you do not need to use the phpbb 2.0.15 update.

I have fixed 4 sites today where people made these errors. My advice when doing these things BACKUP your site and db before starting then go slow and THINK!