@ mario
Please see this FAQ on Why is nothing being imported?. The second paragraph explains what is happening and why and how to address it.
Also, try the beta of the CSV Import one of the major improvements in feedback doing the import process. It also include a code tweak to workaround this common issue.
To receive the beta navigate to the Connections : Settings admin page and click the the Beta tab. Opt in to the CSV Import beta. Next navigate to the WordPress Updates admin page and click the “Check Again” button. That should force WordPress to check for updates for the CSV Import and present it as an option allowing you to update to it. If it does not show as an option, you may have to wait an hour before WordPress allows for the update check to happen again. After it does, the update will display on the WordPress Plugins admin page.
Hope this helps!