First , get this:
Auto Cookies MOD
then, unzip it to your computer,
then create a folder named "install" on your server, inside your phpbb folder.
then upload the cookie.php file to that folder,
then , in your browser, go to:
then click the button to accept the settings it finds.
then delete the install folder.
Then you and all your members MUST
do the cookie dance:
clear your browser cookie
clear your browser cache/temporary internet files
close your browser
go back and test
hold one foot in the air and spin around three time
singing "Do the Cookie Dance, yeah, yeah, yeah."