Here's how to fix this issue:
- Close all Office programs.
- While holding down the Option key, click on the Go menu and choose Library. Your hidden user Library folder opens.
- Navigate to ~/Library/Group Containers/UBF8T346G9.Office/User Content/Startup and open the Word, Excel and PowerPoint folders. Remove the files in each folder (an .xlam for Excel, a .dotm for Word and a .ppam for PowerPoint.
- Restart the Office programs to check that the error message no longer appears.