• https://answers.microsoft.com/en-us/msoffice/forum/msoffice_excel-mso_mac-mso_mac2016/excel-for-mac-opens-all-excel-files/aa648d63-e619-4869-94a0-ce541656ee68?page=1

Please try the following steps to see if the issue can be fixed:

1. Quit all of your Office applications.
2. Clear Office credential caches:
   1. Quit Office applications and open Finder > Applications > Utilities > Keychain Access
   2. Remove the following:
      • Microsoft office Identities Cache 2
      • Microsoft Office Identities Settings 2
3. Remove activation files
   1. Go to the Library folder and open Group Containers. Ctrl+Click each of these folders if present, and Move to Trash.
      • UBF8T346G9.ms
      • UBF8T346G9.Office
      • UBF8T346G9.OfficeOsfWebHost
   2. Restart your mac, open Excel 2016 for Mac and sign in with your account.

Outlook data will be removed when you move the three folders listed in this step to Trash. You should back up these folders before you delete them.