The following entry can be seen in the log file:
Client error: Insufficient system resources exist to complete the requested service
Failed to write data to the file [\\xxx.xx.xxx.xxx\xxx\xxx\veeambackup.vbk].
The error is coming from the Windows server, on which Veeam Backup and Replication is installed.
To resolve this issue, refer to the Microsoft KB that provides instruction on how to address: http://support.microsoft.com/kb/304101