When doing a test payment at checkout (using Stripe) after clicking 'pay' I get the following Internal server error;

("Failed to render template <Template memory:7f8a9b863470> using values {'format_date': <function MailTemplate._render_template.<locals>.<lambda> at 0x7f8a9b309e18>, 'format_datetime': <function MailTemplate._render_template.<locals>.<lambda> at 0x7f8a9b309ea0>, 'format_amount': <function MailTemplate._render_template.<locals>.<lambda> at 0x7f8a9b309730>, 'format_duration': <function MailTemplate._render_template.<locals>.<lambda> at 0x7f8a9b309a60>, 'user': res.users(1,), 'ctx': {'lang': 'en_GB', 'tz': 'Africa/Johannesburg', 'uid': 8, 'force_send': True, 'default_mass_mailing_name': False, 'default_mass_mailing_id': False, 'active_id': 21, 'active_ids': [21], 'active_model': 'sale.order', 'default_composition_mode': 'comment', 'default_model': 'sale.order', 'default_res_id': 21, 'default_template_id': 11, 'custom_layout': 'mail.mail_notification_paynow', 'tpl_partners_only': True, 'safe': False}, 'object': sale.order(21,)}\n\nUndefinedError: 'loop' is undefined", '')