Emails not sent

Hi,

We are using the latest version of the plugin (v1.0) and the latest version of WordPress.
Now we cannot send mass emails anymore. We get this message even though we do specify all form fields, including the email body field :
// 20170505115816
// https://xxx/wp-admin/admin.php?page=Mass-Email

{
“success”: false,
“message”: “Please enter a message.”
}