The configuration variables $g_string_cookie, $g_project_cookie, $g_view_all_cookie, $g_manage_cookie are calculated based on $g_cookie_prefix. When you change the cookie prefix in config_inc.php, you need to follow it with a copy of the four lines that calculate the names for these cookies.
This is specifies to the path under which a cookie is visible. All scripts in this directory and its sub-directories will be able to access Mantis cookies. Default value is '/'. It is recommended to set this to the actual Mantis path.
Unused
Cookie version is used as a prefix for cookies that should be expire when the code is changed in a certain way. The developers would increase this version when necessary, which in effect will cause the creation of new cookies that are compatible with the new code. It is not expected for the user to need to change this setting.
This should be set to a unique identifier which does not include spaces. Again, this should be unique per Mantis installation, specially if the $g_cookie_path is not restricting the cookies scope to the actual Mantis directory.