When looking at the SharePoint ULS logs, it might produce a message similar to the following might occur

[#1]

Issue

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    

 

Resolution

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 :
http://help.nintex.com/en-US/nintex2013/help/Workflow/RootCategory/Designer/Nintex.Workflow.UsingExt...
and
https://community.nintex.com/community/tech-blog/blog/2016/01/20/external-start-feature-for-nintex-w...

•  if you are not going to use the external workflow start feature, then i would recommend that you stop the service "Nintex Workflow Start" on the Central Administration.
• But if you wanted to keep it, but need this message to be rectified, then you would need start it from the Central Administration page.
• Make sure your SharePoint server have internet connection, and the settings in proxy or firewall in your system doesn't block the connection of this service.