You are using Internet Explorer? You might try making sure you have it selected as the default browser in Windows settings. That can often solve this 0Auth issue. If that does not work, try selecting another web browser as the default.
If you still have no luck, you should open a support ticket with eM Client.
Thanks Gary. Another expert, on the google forum, suggested that I should upgrade eM Client from version 6 to 7, and it worked.