Newsletter plugin upgrade generate “Serial is not valid” message

I upgraded the Newsletter plugin to the latest version 4.6.7.4 and this led my wordpress site to be inaccessible, even the admin page, only showing the “Serial is not valid” message. I am not using any PRO version, so I don’t understand why I am getting this. I had to disable all plugins via phpmyadmin, and reenable them one by one…