The cause appears to be the use of non-ASCII characters in file names. Microsoft describes it like so: “Any user may be ...

Microsoft has started looking into a problem that is preventing users from opening some Excel file attachments with non-ASCII ...

There is a workaround, though.

If you see the “The file couldn’t open in protected view” error while opening Microsoft Excel, the solutions listed in this post may help you resolve the issue ...

Here, in this section of the post, we will recommend the five most effective tools and methods to repair the Excel file. Microsoft Excel’s Native Repairing Feature Online File Repair Service Recovery ...

Microsoft is working to resolve a known issue that prevents some users from opening Excel email attachments in the new Outlook client.