new-mailboxexport batch randomly fails

Hi all
we're using a powershell script to export our mailboxes from 3 databases to .PST files.

Here's the problematic portion:

$ExportShare = \\localhost\i$\somefolder
foreach ($Mailbox in $Mailboxes)
{
New-MailboxExportRequest -BadItemLimit 100 -BatchName $BatchName -Mailbox $Mailbox -FilePath "$($ExportShare)\$($Mailbox).PST"
}

It does the job and reports that export was complited (issueing get-mailboxexportrequest) for most of mailboxes,  but some mailbox.PST files are of zero size and some request were failing. 
We're also generating export report like this:

# Write reports if required
if ($ReportShare)
{
Write-Output "Writing reports to $($ReportShare)"
$Completed2 = Get-MailboxExportRequest -BatchName $BatchName2 | Where {$_.Status -eq "Completed"} | Get-MailboxExportRequestStatistics | Format-List
if ($Completed2)
{
$Completed2 | Out-File -FilePath "$($ReportShare)\$($BatchName2)_Completed_Second_try.txt"
}
$Incomplete2 = Get-MailboxExportRequest -BatchName $BatchName2 | Where {$_.Status -ne "Completed"} | Get-MailboxExportRequestStatistics | Format-List
if ($Incomplete2)
{
$Incomplete2 | Out-File -FilePath "$($ReportShare)\$($BatchName2)_Incomplete_Report_Second_try.txt"
}
}


And in those reports for zero sized mailboxes we see the following:

FailureType                            : PSTIOExceptionTransientException
FailureSide                            : Target
Message                                : Error: The server or share name specified in the path may be invalid, or the 
                                         file could be locked. --> The specified network name is no longer available.


Looks like there was some problem with the share we're writing to the PST files. But the share is \\localhost\i$\somefolder and it is an iscsi storage, I checked logs on the side of the storage - everything
looks clean.
We're running the script under domain administrator account, and most of mailboxes are exported successfully.

Googling for PSTIOExceptionTransientException and the message doesn't give anything.