When trying to perform a File Backup job using Veeam Backup and Replication (specifically version 10.0.0.4461) it would randomly fail with An unexpected network error occurred. This backup job was configured to backup a local NFS File Share to a local SMB (CIFS) backup repository. Repeating the job would result in another failure but differing amounts of data being transferred each time. After the failure, the job report listed items such as NasMaster.ExecuteBackupProcessor and boost::filesystem::status:.
It continued to fail even after setting the Backup I/O control of the NFS File Share to the lowest impact value. To resolve it the backup repository was switched from SMB/CIFS to NFS. After this change the Backup I/O control setting was also reset back to the default (middle) value. This solution likely indicates a problem with the destination (backup repository), so if you need to keep it as SMB/CIFS then you can try adjusting the Load Control settings of the repository (right click Properties menu -> Repository). For example try reducing the maximum number of concurrent tasks. If it fails with just 1 task then you may also need to try limiting the data rate.