The following message might be found in the SharePoint ULS logs :
Nintex.Workflow.Start.Service.e (0x0604) 0x0620 Nintex Workflow Start Service Information 00000 Verbose Calling API to get farm connection string
This service "Nintex Workflow Start" service is installed during Nintex workflow installation.
you should find it as following : On the Central Administration Home page, click System Settings, and then click Manage services on server.
see this link :