1) Do you have Safari set to accept cookies from anywhere in the prefs section?
Preferences > Security icon > Accept Cookies
Choose: Always (most convenient)
2) Did you try setting this to a slash in config.inc.php?
"If you are using cookies you can specify a path as well. Leaving this blank will make the cookies only available under the UBB.threads install directory. Setting this to / will make them available to your entire website."
Just a wild stab at it