If this error is reported during the WorkgroupShare client setup, right after selecting the user account and entering the password, then this is most likely to relate to other add-ins installed into Outlook. It occurs because Outlook is not responding correctly when the setup tries to view the list of folders that can be shared. Please refer to the following FAQ How do I resolve Add-in Conflicts?
If this does not help to resolve the issue then please make sure that you are running the latest release WorkgroupShare. Click this link for more information. When running the client setup, and when selecting the user account, choose to not select folders to share. Once setup is complete you can then open Outlook and share folders by clicking Tools, Share, Settings.
If applicable, please also refer to the following FAQ
Client setup fails with the message ‘The server is not available. Contact your administrator if this condition persists’