Just had this problem today and found a relatively simple fix for it – run Office 365 Quick Repair!
If you search this problem you will also find this suggestion for changing a setting in Excel, I did NOT have to do this:
1) Open Excel > click on File > Options > section Advanced.
2) Under General section > select the checkbox for Ignore other applications that use Dynamic Data Exchange (DDE) > OK.
3) Now try opening the Excel file from Dropbox and verify the result.