Topic
When sending a web request using the Web Request workflow action to a web service that requires, or expects, line breaks in the request body returns a 400 'Bad Request' response.
Instructions
Update to the following versions of Nintex for SharePoint, or later:
Nintex for SharePoint 2013: 3.6.7.10
Nintex for SharePoint 2016: 4.6.7.10
Nintex for SharePoint 2019: 5.2.7.10
Nintex for SharePoint Subscription Edition: 6.0.3.10
Additional Information
The root issue was that the web request body was being encoded. This would result in the web service receiving an invalid request body, and not responding as expected.
Note: This issue pertains to a specific scenario. Not all 400 responses from web requests can be attributed to this one issue or Nintex Workflow. 400 responses have an infinite number of possible causes. If your web request is failing in Nintex Workflow, proceed to test your web request in another application such as Postman or PowerShell to troubleshoot further.