This error happens because of a malformed image URL. One of the vertex images may have a name which contains not allowed characters for a path and when NodeXL tries to download that image to display it, you get this error. You can avoid this error for now by telling NodeXL to not get vertex images.
Please send the workbook to info@smrfoundation.org after you collect the data so we can investigate which image it is and we will try to resolve it.