I’ve been battling to get SSI working for some time now when I had my forum in a subdomain, ie http://forum.smartphonesoft.com. Fortunately I came across this information, which fixed it for me. The actual fix from that site is as follows:
Create a "vhost.conf" file in "/var/www/vhosts//conf/"

php_admin_value safe_mode off
php_admin_value open_basedir none

Rebuild the domain configs for the particular host via:
/usr/local/psa/admin/sbin/websrvmng -u --vhost-name=
or rebuild all via:
# /usr/local/psa/admin/sbin/websrvmng -a