SSI Exec no longer works under https

We use a lot of SSI on our site. exec include are not firing under https:

see

[[https://www.himalayanacademy.com/donations/donate_publications.shtml]]

#This one works (the footer is cat'ed into the page as expected):

https//

# This one does not (it calls set of side bar links based on the location of the page):

It *used* to work under PLESK set up... so I suspect that there is some setting that is blocking cgi execution under SSL.. where do I go to fix that?