This problem happened to me two times already in two different libraries!
I checked the option 'Enable workflow to start from the item menu' in the workflow settings.
Then I disabled a previous version in the workflow settings (List --> Workflow Settings (above Create Workflow with Nintex Workflow)). I don't know if that had something to do with it though.
Then I had the name of the workflow 2 times in my item menu.
If I uncheck the box at 'Enable the workflow to start from the item menu' just one of the two disappears and the second one doesn't work (something like wrong page, return to start page)
Someone with the same problem who knows what to do? Thanks!
I use SharePoint and Nintex 2013