Add media not working on newsletter

I have a custom role for some users on my site called 'Staff/Contractor'. I have ticked all options under e-Newsletter settings for this role so that they have full access to use the newsletters. When editing a newsletter, they cannot click 'Add media'. The window flashes up for a split second and then disappears and cannot be brought up.

This problem does not happen for admin when editing newsletters. This problem does not happen for Staff/Contractor role when using a page or post and trying to add media.

Please advise?

Also, I would appreciate if someone could reply to my other ticket, I understand you might be busy, but I would appreciate a reply, even if it just that you are looking into it: https://premium.wpmudev.org/forums/topic/e-newsletter-doesnt-seem-to-be-syncing-with-my-users-to-update-email-addresses?replies=7#post-846662

Thank you