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 (Vista, 7, 8 or 10) the maximum number is considerably lower than on server versions of Windows.

The following Knowledge Base article may also be useful: