We weren't able to narrow it down to a specific material. But any of the objects using an xref material were fine.
We've also discovered that the broken file render eventually and can get "stuck" at different points, on some computers it is at transforming vertices, at others it is building acc., etc.
I'm uploading the scene to the helpdesk now. It is able to render, but it takes an age to get to the pass stage. And maneuvering in the view port is nearly impossible because it is so laggy. Also compared to vray the file takes noticeably longer to open.