My best guess is that this is caused by Nintex and Sharepoint running on different batches, so it actually updates the name AFTER you copy the document to the destination library. What you can try in the workflow is to put a “Commit Pending Changes” action in between the renaming of the file and the action that sends the document to the destination library (I'm assuming from the other post that'll be the Webservice Query). Some actions like updating item fields are only actually executed once the workflow reaches a certain “commit” point, these can be either the workflow pausing for some reason (like assigning a task or a direct pause action), the workflow ending, or a Commit Pending Changes action. Putting the Commit Pending Changes action in between will force the system to update the name before it goes on to copy the file to the destination library. See also this explanation

Thank you for the reply (again).  I have solved. 

In this case, there were multiple docs inside the same docset - the WF changes the name of the document (used a string saved to a variable) to set the new name.  The WF correctly routed that PDF to the final destination and as it went to the next document, it errored - I believe because the new name has already been given/used (by the first doc).  So, I added the ID to the end of the new name string (unique identifier) and vwaaa la, no more wf errors.   This is a site WF kicking off a doclib WF and it all worked as intended.