This issue is a known bug and was reported with issue 1152. Scheduled to be released with SPE 6.1.
The quickest way to get a fix is to apply the following changes to the script:
/sitecore/system/Modules/PowerShell/Script Library/SPE/Tools/Data Management/Toolbox/Data Management/Data Importer
$itemPath = "$($destinationNode.ItemPath)/$($row.Name)"
$itemPath = "$($destinationNode.ProviderPath)\$($row.Name)"
Alternatively, you can copy all of the script from the yml on the master branch as it addresses some other issues as well.