Since last update, I get an error message "données WordPress : [Unknown collation: 'utf8']"... So I think it's better for me to trash MailPoet. Don't understand !!
I finally solved the issue.
The line [define('DB_COLLATE', '');] was set to 'utf8-general-ci'
I restored it at its default value ('') then all rocks.
The plugins works well now, but the its use should be more intuitive.