Hi Stefano,
I tried what you said, I also changed your code after finding something in the docs. I used both codes but they give me the same error, see below:
$subscriber = Newsletter::instance()->get_user($user_id);
$subscriber->list_2 = '1';
Newsletter::instance()->save_user($subscriber);
Also when I first declare the newsletter as object it isn't working. You said it had to be get_user_by_wp_user_id() but the documentation says get_user, but both methods work.
I get the following error when approving the user:
Database error. If you were saving a newsletter try a table upgrade from the status panel.
I hope you see what's happening,
Kind regards,
Mark