So when we click on an item in SharePoint (onprem) the item comes up fine but when going to the 'workflows' link it fails saying that it can't find a column 'Approval3' on the list that the workflow was running on.
The 'workflow link' referred to is in the ribbon:
If you go to 'Manage Workflow Inventory' you'll see a list of all workflows on your site. At the far right there is a delete action. It was from here that the workflow was deleted. Clicking on the workflow link throws an error which is recorded in the ULS logs:
System.ArgumentException: Column 'Approval3' does not exist. It may have been deleted by another user.
Approval3 is not a column that was created by a user. I think it is a workflow status column that gets created when a workflow is either published or is executed for the first time. When the workflow was deleted it apparently removed this column.
Any ideas how to resolve this situation?
So what happened was due to the large size of the list the operations that normally run when deleting a workflow from the inventory page were not able to finish and this led to a situation where workflows were still running on items that no longer had a workflow instance to run on them. We cancelled the workflows on those items via powershell and restarted it. This resolved the issue for us.