Issue with Nintex live not getting forms through
From December 1st 2020, Nintex will update the Live Router to remove support for Legacy Transport Layer Security (TLS) 1.0, 1.1. If any Nintex Live services are in use, you will be required to update your SharePoint deployment to support TLS 1.2 to ensure that the Live services run properly. All Nintex for SharePoint (2010, 2013, 2016, 2019) platforms and their respective Live components will be affected.
The fix is to update the registry on all SP servers (https://community.nintex.com/t5/Technical-Issues/Workflow-fails-when-connecting-to-the-Nintex-External-Platform/ta-p/118012)
The next step is to apply this registry change to production (4 servers):
Export existing registry configuration
Import new registry key
(repeat on all servers)