How to fix ADD-IN ERROR in Microsoft Word that says 'You do not have permission to view this directory or page using the credentials that you supplied'

Question

Every time the user uses Microsoft Word Add-in 'Icertis Experience for Word' error message below is encountered

Below are the things done to resolve the issue but to no avail:

1. Check the settings in Trust Center (everything ok)
2. Deleted all files in folders below:
   1. %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\
      1. %temp
3. Uninstalled and installed the plug-in
4. Reinstalled Microsoft Word via online repair and deleted local cache credentials of office.