Hi Luca, I ran into this problem as well and found the answer in another discussion - They basically explained that once a mendix app is published, then all data created locally is erased.
However, you can still use the 'demo_administrator' and 'demo_user' demo user credentials to sign in to your published app: Go to app settings > Security > Demo users > Select one of the demo users and select 'copy password to clipboard' to use as the demo user's password when signing in. Hope that helps!