I'm having exactly the same problem, but I stumbled across this whilst researching this particular issue.
https://productforums.google.com/forum/#!topic/drive/tLVhGbITFMk
It suggests that there is a 'partial sign out issue' with a Google Account. Having read the doc above and tested being signed in and signed out of my google account, it seems to have solved the problem.
It's an imperfect solution I grant, but as no definitive error code is thrown due to the security constraints of clickjacking, I get it.
It may continue to present problems to your end users, but as mine are local and limited to a maximum of 3, it isn't a huge issue to overcome with a little polite coercion.
-- FURTHER Information --
It looks like the 'solution' was a fluke and it has since been confirmed by Google that this feature is actively being discontinued. (thanks to: @omniasoft (twitter) for posing the question to Google.
Google Docs viewer returning 204 responses, no longer working, alternatives?
https://productforums.google.com/forum/#!msg/docs/hmj39HMDP1M/X6a8xJwLBQAJ
Sorry for any false hope!