Had this issue where a document workflow does not want to start when the document is edited and there is a conditional start in play.
Got this in the ULS logs:
Error processing item updated event.: System.ArgumentException: New instances of this workflow template are currently disallowed.
We renamed the Nintex workflow a while ago and that changes the workflow with the old name to unpublished.
Delete the workflow with the old name from "Unpublished", this fixed the error for me.