I also noticed a good thing to do, after any changes with cookies etc, is to empty the session cookies via myphpadmin on your host...
on the database where the phpbb is installed
Not sure if relevant, but it seemed to work for me
my login & log out seem to work fine.
so here are the settings i currently have
Cookie domain = www.etc.com
Cookie Name = phpbb2mysql
Cookie Path = /
Hope it helps in some small way