Object Crawl-0 failed in event OnBackup.

I am trying to do a full backup via task scheduler

with this script:

$ver = $host | select version
if ($ver.Version.Major -gt 1) {$host.Runspace.ThreadOptions = "ReuseThread"} 
if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null) {
    Add-PSSnapin "Microsoft.SharePoint.PowerShell"
}
Backup-SPFarm -Directory \\11.11.11.111\nasbk02\BKPORTAL -BackupMethod full -BackupThreads 10 -Force

but I am always getting this error:

Object Crawl-0 (C: on APPUNICOOP01) failed in event OnBackup.