Clearing Cache on Desktop Outlook Add in
1. Close Outlook app first.
2. From the PC, Click on Search then type or select Run.
3. Enter this in the box %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\ then click OK.
4. A pop up will show up, look for the webview2 folder and delete this completely.
5. Open Outlook app and try linking emails again using the add in
If you are using Web Outlook and has the same issue, you can also clear your browser cache. Please see steps: Clearing Cache in Browsers