It sounds like the privacy check is returning an unexpected result.
There are two ways you could check this.
a) In /admin/site-map, click ACS Kernel under Services, locate the PrivacyControlEnabledP parameter and set it to zero.
b) In /packages/forums-portlet/www/forums-portlet.tcl, comment out this line
set can_read_private_data_p [acs_privacy::user_can_read_private_data_p -object_id [ad_conn package_id]]
and temporarily replace it with this one
set can_read_private_data_p 1
Do either of these make the forum portlet show up?
Thanks,
Andrew