The error might be reported during backup.
the computer name or IP address has been changed on the backup source computer or it cannot be resolved from the DS-Client computer.
One more reason for this issue:
Ensure that the backup source computer name can be resolved from the DS-Client computer (e.g. try to create in DS-User a file system set and place the string \\source-computer-name in the target server filed or expand Microsoft Windows Network and try to select the backup source computer from the servers list).
Check also the Microsoft KB: