Hi Lage,
sorry for those troubles. We went back to developing this migration plugin after long break and we know that not everything works. That’s why we strongly suggest to have db backup.
Anyway, if you would like to help for us to fix those problems, please create ticket in our forum here https://wpml.org/forums/ , just create quick post with link to this forum thread and mention my name (Konrad) so this will be assigned to me and I will be able to check what went wrong.