As far as best practices are concerned, should user be able to scroll the page while a modal is active?
The entire point of a modal is that the user needs to focus on that task before doing anything else.
Modality is most appropriate when:
It’s critical to get the user’s attention.
A task must be completed (or explicitly abandoned) to avoid leaving the user’s data in an ambiguous state.
Considering this, I would say it would be counter intuitive to let the user perform any action outside of the modal. If you still feel the need for letting the user work in the background, then you should re-evaluate the need for a modal.
I pulled the guideline for modal from the iOS HIG, but, it is applicable even in web scenario.
If there is scrollable content within the modal dialog box, then no, scrolling should be disabled in the main window. (This is to avoid double scrolling from the user having their cursor in the wrong place.)
If there is no scrollable content, then I think it depends on the context. If there's something asking me to sign up or preventing me from proceeding without taking action in the modal, being able to scroll is nice. It allows users to get a "sneak peek" of what they can see if they were to follow your CTA.