Hi Alessandro,
I signed up for an account at ServerPlan and setup a copy of mojo 2.9.0.1. I followed the same steps you listed and I also added the Let's Encrypt certificate. I do not have the UseSSLForLDAP setting set to true.
I am able to login to the site with the default login.
Were there any other settings you configured in the web.config or user.config before trying to login?