I've encountered the same issue and stumbled upon some interesting findings during my research on this topic.
Check out this link for more insights
After reading through the suggested solution regarding a 'partial sign out issue' with Google Account, I experimented with signing in and out of my account which surprisingly resolved the problem.
Although not a perfect fix, considering that no error code is provided due to security limitations, I understand the situation well.
You might still face challenges with your end users, but since mine are local and limited to just 3, it's manageable with some gentle persuasion.
-- ADDITIONAL Information --
It appears that the temporary 'solution' turned out to be accidental, as Google has officially announced the discontinuation of this feature. (credits to: @omniasoft on Twitter for raising the question to Google).
Looking for alternatives to the non-functional Google Docs viewer?
Here is another useful link for reference.
Apologies if this information led to any false hope!