When a workflow with a flexi task action fails with "Nintex Flexi Task Error: Error in task. Cannot find a corresponding human workflow task id for this task" and a task item is not created in the target task list.

This error usually is the result of the workflow progress table being purged after a task item is created but before the assignee attempts to open and respond to the task.  In this case the error is thrown when the workflow attempt to create the task item.  In the ULS Logs you will see the following:
Invalid field name. {17ca3a22-fdfe-46eb-99b5-9646baed3f16} /<site>/Lists/Workflow Tasks

This particular error indicates an issue with a missing or corrupt SharePoint feature.

Resolution:

Execute the following PowerShell command

Disable-SPFeature –identity "Fields" -URL RootWebURL
Enable-SPFeature –identity "Fields" -URL RootWebURL