Ralph Asset Management System - User Group

Import from xlsx file - error

Hello,
I have problem with import user data from file,
I exported user list, added new users and try import,
but
I have errors,
f.ex. KeyError encountred while trying to read file: ‘Unknown relationship: None’
what it mean ?
I tryed different options, cvs, xls, xlsx, nothing work,
what I am doing wrong ?
regards