There was an issue i had encountered today while starting workflow using Nintex. The following thread helped me.
While trying to start the workflow, it is failing while "Assigning Task to Apprver" basically in Nintex Language at "Request Approval" action item.
The issue is the title contains some hidden characters. I copied the title and pasted in Notepad and found that there is one Escape character which is causing the workflow to fail. I cleaned up that character and after that workflow started working fine.
Simple Fix! Isn't it?