Thanks to : https://www.outlook-apps.com/inactive-outlook-add-ins/ For me it was sufficient to do the following: 1) Close Outlook. 2) Remove the key in the registry: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Resiliency\DisabledItems . Find the one which links to UCAddin.dll and remove it. 3) Start Outlook again. 4) Enable the Add-in like File -> Options -> Add Ins -> Go -> Check the Skype Add in. (The Load Behaviour should say "Load at startup" now.