The error may be reported when running backups for backup sets configured with VSS option, or for backup sets protecting System State and Services Database ( the VSS option will be used even if not selected when backing up the system state of computers running Windows Vista/2008 or newer ).
Usually this error comes together with other error messages, like: - Access is denied. (\\ICBSVR001\ADMIN$\complusbackup\vsscontrolin.dat) (End Snap) - Access is denied. (Scheduler - "C:\Windows\complusbackup\sfcfiles.exe")
During backup the DS-Client application sends keep alive messages to the backup source in order to keep the connection open until all the required backup data is retrieved, if the backup target computer is very busy and does not respond to these keep-alive messages for more than 300 seconds the error is logged in DS-User event log and the connection to the backup source closed.
If the steps / resolutions provided for the other error messages that come together with this one do not help or are not applicable try to increase the value for the following parameter: DS-User > Setup > Configuration > Advanced > Category: Communication, parameter name: VSSKeepAliveTimeout, set it to 600 (default is 300).