The problem is in my file settings.php (which renders the form). Look at the form action. It says action="options.php" and then the settings_field() inserts a hidden field with the correct action. But WordPress prioritizes the form action, so, when submit the form, WordPress tries to load options.php instead of the correct action in the hidden field.
The solution? Simple:
Replace in the form action="options.php" by action="".
This simple patch fixes the error.