We had the same issue too many complaints from our customers. We traced it to the Excel OFffice Security patch KB3115262 - https://support.microsoft.com/en-us/kb/3115262 Which came out July 12, 2016
To work around the issue, we had customers make a change in their Excel to allow files from the internet.
To fix:
1) Open Excel Go to File Options
2) Click Trust Center -> Trust Center Settings
3) Go to Protected View. there are 3 options that show that were all clicked
We uncheck the first option that reads -- "Enable Protected View for files originating from the Internet"
That fixed the issue. Perhaps not the best solution. I'm not sure why this particular KB broke this but I think perhaps the fact the file format is not expected and this setting are conflicting with each other.
On a computer I have that doesn't have this KB installed, all those are checked and it still works fine (not blank but prompts file format is different) and I think shows in protected view.
What led us to fact its not the content of the file is we noticed if we resave the file in notepad or notepad++ without making any changes, the file behaves fine so excel must be reading some property of the file rather than the content to block it.
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