The NintexWorkflows library for a given site is at the following URL.
http://[mysitename]/NintexWorkflows/Forms/AllItems.aspx
where [mysitename] is the URL for your site.
Check this library if you receive an unexpected error for the name of a new or imported workflow. If that workflow name is not listed on the Workflow Gallery page, then an orphaned workflow is likely. A workflow is orphaned when its associated list or library is deleted. Orphaned workflows remain in the NintexWorkflows library, along with any active workflows.
To remove orphaned workflows from the site and make their names available for reuse, delete them from the NintexWorkflows library.