My workflow was working sweet as a nut then I started getting the error message on a flexi task. I Googled the error and tried adding a New Task in the workflow settings but I got a slightly different 'null' error.
I tried a few other things then set up a test library and test workflow and I get the same error. Is it the environment? Any suggestions on what I can do?
I found this which makes sense, but no suggestions of a solution, in my case the second cause might be the issue. I'm using SharePoint 2016:
You find the error "Cannot find a corresponding human workflow task ID for this task" when clicking a link in a workflow email notification. When the user attempts to view the task, the page queries the Nintex Workflow database for the record linked to the task item. When a record is not found, this error message is displayed.
There are a couple of reasons why the data can be missing from the Nintex Workflow database:
1. Data has been manually purged from the database during a maintenance operation.
2. Database or SharePoint migration has resulted in the site collection accessing the wrong database, or a site collection ID has changed so it does not match the recorded data.
3. A workflow has restarted after timing out while trying to commit at run time.
The third case is relevant to performance and workflow size. If a workflow fails to execute due to being too large, SharePoint will attempt to run it again from the beginning. While the workflow was running, a task notification was sent to a user, a task item was created in SharePoint and records to track the task were recorded in the Nintex Workflow database.
When the workflow restarts, the workflow detects this and clears any existing records from the database so the status tracking can begin again. However, the original task item still exists in SharePoint, so if a user uses a hyperlink from the original notification, they will see a task with no backing data in the database. This will result in the error "Cannot find a corresponding human workflow task ID for this task".