If the Word Add-in and/or Send to Case are not loading, clear the Office cache by taking the following steps:
First, close all Word documents if the Word Add-in is not loading and/or Outlook if Send to case is not loading.
For Windows, copy the following path %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\ and paste the path in the Type here to search box next to the Windows Start button. Hit ENTER:
In the folder that opens, use CTRL+A to select all the folders present and Delete them:
Close the window and open a fresh Word document or Outlook to confirm that Word Add-in and/or the Send to Case are loading.
Information from the following link has been used to create this Knowledge Base article: https://docs.microsoft.com/en-us/office/dev/add-ins/testing/clear-cache