I seem to recall some issue with session id and phpbb, although I'm unable to find the thread right offhand.
What is the name of the session id that phpbb uses? You might need to modify this bit of code in config.php for that:
Code:
define('PHPDIG_SESSID_VAR','PHPSESSID,s'); // name of SID or variable to remove
// can be 's' or comma delimited 's,id,var,foo,etc'
Hope this helps.