Hi Libi ,
Apologies for any inconvenience caused.
That error is triggered because the WC()->mailer()->get_emails() function isn't returning what it should. It should return an array of WC_Email objects, but the array includes at least an integer.
That means that one (or more) of your installed plugins use the 'woocommerce_email_classes' hook wrongly. You should check that.
Even so, we've added a check to add only items with the correct class. The change will be included in the next ATUM version.
Best Regards,
José Andrés