As far as your cookie question since you posted while I was replying
Your domain can be empty if you want although we recommend .domain.com because that covers the entire domain.
what happens if someone logins on the forum at www.domain.com
and your gallery is on domain.com the person would not be logged in because those are two different domains.
Now if this is a database you used elsewhere maybe on a different domain etc just guessing use phpmyadmin check your database pp_settings table settings 171-173 and see what the cookies are set to.
Like I said if I can not see what your stuff is setup to hard for me to say other than to list the obvious.