I'm sorry, but this may only be a problem on the client side. We often have problems with Office and we have to repair Office, reconnect the plug-in or, in more serious situations, recreate your user profile on your machine.
If this add-in requires a service, a feature or a solution on the SharePoint side, you can try uninstalling / retracting / etc. and then redeploy. Patches can also help this type of problem, but usually not.
If you provide more information on how the plug-in is installed / implemented for users, what are their requirements (server side and client side), we may be able to provide you with better advice.