Powershell Issue with change Password

Can't test it at the moment, but I strongly suspect that $account.psbase.invoke("setpassword", $newPassword) returns $null, since you're not assigning or voiding it. $account is a DirectoryEntry object, which can behave strangely. When that $null hits Export-Csv, the cmdlet doesn't know what to do with it.
That aside, your output objects have inconsistent properties, and you're referencing an undefined variable $Status.
There's also no need to store the results; use the ForEach-Object cmdlet and pipe the results directly to Export-Csv.
Try something like this:

$inFile = "g:\computers.txt"
$outFile = "g:\password.csv"
Function Get-RandomPassword {
    Param (
        [Parameter(Mandatory)]
        [int]$Length,
        [int]$AmountOfNonAlphanumeric = 1
    )
    Add-Type -AssemblyName 'System.Web'
    [System.Web.Security.Membership]::GeneratePassword($Length, $AmountOfNonAlphanumeric)
}
Get-Content -Path $inFile -ErrorAction Stop | ForEach-Object {
    $computer = $
    Write-Host "Processing $($computer)"
    $result = [PSCustomObject]@{
        ComputerName   = $computer
        Status         = 'OFFLINE'
        Error          = $null
        NewCredentials = $null
    }
    If ((Test-Connection -ComputerName $computer -Count 1 -ErrorAction SilentlyContinue -ErrorVariable testError)) {
        Try {
            $newPassword = Get-RandomPassword -Length 12
            $account = [ADSI]("WinNT://$($computer)/Administrator")
            [void]$account.psbase.Invoke('setpassword', $newPassword)
            $result.Status = 'OK'
            $result.NewCredentials = $newPassword
        } Catch {
            $result.Status = 'ERROR'
            $result.Error = $.Exception.Message
        }
    } Else {
        # If Count in Test-Connection will always remain at 1, a simple "$result.Error = $testError[0].Exception.Message" will do.
        # This version will also handle multiple pings:
        $result.Error = ($testError | ForEach-Object {$_.Exception.Message} | Select-Object -Unique) -join '; '
    }
    $result
} | Export-Csv -Path $outFile -NoTypeInformation

Edit: COM object will now be released.
Edit2: $account is not a COM object after all (but DirectoryEntries are not a lot better) ...