You may see an error like this:
The file couldn't open in Protected View
Or the error might look like this:
Word experienced an error trying to open the file
To work around this problem, it seems the only solution is to disable Protected View.
To disable Protected View, go to the upper left corner of Word or Excel, and click File -> Options,
(Options will be near the bottom after you click File)
Click Trust Center on the left side:
Click the Trust Center settings button:
Then Protected View on the left side:
Then un-tick the boxes for, "Enable protected view for files from the Internet," and "Enable protected view for Outlook attachments."
Do not un-tick the middle box regarding files from potentially unsafe locations.
Do not un-tick the middle box regarding files from potentially unsafe locations.
Be very careful going forward, as this does remove one small layer of security: you will no longer have to click the "Enable Editing" button on files you open in 365, so be absolutely sure to not open email attachments you weren't expecting, or from people you don't know, and especially be wary of any Word or Excel files you download from web sites on the Internet.