Page Not Found when Previewing Post for Non-Admins

All users below admin level receive a “Page Not Found” 404 error when previewing posts on my website. http://abn.org.au and test site http://staging.abn.org.au. I have about 20 contributors who all have this problem and I have tried logging in as them, and I can see that the feature is broken.

Can you advise any plugin or settings update I need to make to the site to enable this feature? Can’t find much documentation about this. Any help much appreciated.