Invalid email configuration

If you save the settings the message should go away when you view the form again.

I'm seeing this issue when trying to set a default value on the field. The settings form cannot be saved with a default value.

Are you also using 2.2.2? If so, can you try the latest version?

I am using the latest version.

What if you go to the "Manage form display" page and change the widget of the subscription field to "subscription form"?