This error may occur because you have exceeded the maximum number of allowed connections with the remote computer. For example, if you are making a backup to a server version of Windows, e.g. Windows 2003, and that server has a maximum number of client connections (based on your licensing) then if you exceed that number of connections you may get this error. With desktop versions of Windows (XP, Vista, 7, and 8) the maximum number is considerably lower than on server versions of Windows.
The following Knowledge Base article may also be useful: