Thank you for your response.
I upgraded from one version earlier to 2.1.0, it might be 2.0.22. I use Bank Details as a payment method.
I’ve edited the Display.php file and change this:
$due_date = $invoice_info->display('nextDate');
into:
$due_date = $invoice_info->display('due_date');
And now the Next Due Date section shows the invoices’ due date (not today’s date anymore).
But I’m not sure whether this is a “solution” or not. There’s still a problem sorting the invoices based on due date.