Thank you for your answer.
Yes, Ninja Forms is shown as “activated” on the plugins page.
Unfortunately, even if I deactivate every single other plugin (there are just 3 other plugins used in my site), the menu item for Ninja Forms won’t show up.
If I manually call the URL of the Ninja Forms configuration from the previous version, I get an error saying that I am not authorized to see this page.
This problem is really frustrating to me because we don’t get any error or any other hint ??