Make sure you have setup a Windows account with an Exchange mailbox and you provided valid credentials in the Allocatus Server Configuration dialog. Also verify the authentication method (see http://community.allocatus.com/forums/thread/416.aspx)

In most cases the cause for this error message is that authentication method configured in Allocatus Server and Exchange Server does not match. Please find out if the WebDAV virtual directory Exchange is configured for Basic, NTLM/Integrated Windows or Forms-based authentication and set the value accordingly on the Exchange ...