Go to your PHP Nuke Admin Panel > Click on Fourms > Left side scroll down till U see General Admin > Click on Configuration > Mid way down look for Cookie settings - set them this way:
Cookie Domian = www.yoursite.com
Cookie Name = phpbb2mysql
Cookie Path = /
Cookie Secure = Disable
Session Lengths = 3600
Hit Submit on the bottom of the page...
Good luck and let me know if all is ok..