You're receiving this message because a remote user not registered in your Authorized Users list (Main Menu / Options / Security settings) and Learning Mode is enabled.
If you want to authorize this user to access your Network Clipboard & Viewer, select Ok.
Check the "Add to authorized list" box if you want to add this user to the Authorized Users list (if you click the Ok button, will be added and authorized -checked-, if you click the Cancel button, will be added as unauthorized).
If you don't want to receive this message, you can add user to your user's list or disable the Learning Mode.
If user name is "Unknown User", a remote user NOT LOGGED on Windows is trying to access. For security reasons, you may prefer to deny access and request remote user to login Windows before trying to connect to your Network Clipboard & Viewer.