User can not be found Error Exception in Sharepoint Designer Workflow Publishing

While doing same changes which tested on UAT on a SharePoint production server I found error "User can not be found"
While publishing the workflow,
While clicking on list workflow settings
And While Activating "Workflows" Site collection feature.

Googling through various options I couldn't succeeded then I thought can I import SPD WF from one Farm to other FARM ? and I found following which saved my weekend :)

https://chanakyajayabalan.wordpress.com/2012/01/02/copyingmoving-sharepoint-2010-designer-workflows/

POINTS TO READ : 

1)) I had to re-publish all SPD and Reusable WFs.

2)) I didn't activated claims authentication.

3)) In all workflows variables of lookup type I had to manually re-set. Importing WF lost lookup and showed GUID only.

May be helpful to someone...