403 Permission Denied

Hi there.... first-time WPMUDEV user and question-asker here...

I have a single page (custom post archive, to be specific) on my website that was working perfectly fine, but now brings up 403 Permission Denied when I try to access it. I've tried the usual suspects (deactivating all plugins) and even rest my theme (Headway) but to no avail. I have used the same custom post plugin to create other custom posts on the site, and they still work fine. Any thoughts/pointers would be most appreciated! Thanks.

(Unfortunately can't share link as it's 'under construction' right now.)