I've seem this issue a few times, If it is the same one it was an issue with the backend workflow manager service, I logged a job with Microsoft and they fixed it after a while.

Hi -

Do you know what Microsoft fixed on the backend?  I saw your old post and the link is broken so I can't see the solution.  As I said, we just got Nintex and one of or user added the WF and forms on his SharePoint site and this is the error he is getting. We are trying to fix this as soon as we can.  This is not really good as this is first user that trying to use on his site.

Not sure specifically, Logged a job with Microsoft and they fixed something in the back end system