1. Close SPD2013
2. Delete everything at %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache
3. Delete everything at %APPDATA%\Microsoft\Web Server Extensions\Cache
4. Go to SPD2013 options –> General –> Application Options –> Uncheck the “Cache site data across SharePoint Designer sessions”

Probable solution/findings to verify:

1. Generally occurs with send email functionality in workflow.
2. Check outgoing email settings for the farm are correct.
3. Disabled firewall on the Exchange/SMTP server.
4. Check App management service is running, UPA is set up and  completed sync such that the user running the workflow has a profile.

1) Workflow end point returning the xml- This indicates wf is configured properly
2) Workflow published with Site admin account
3) User profile up and running
4) Workflow published account was coming in user profile search(verified from central admin)
5) No Logging in workflow(Just only one statement in workflow)

Generally I've experienced issues like these with activities involving users:

Sending email
Getting user details (ex: User title, user email address, user principal ID)
Insufficient permissions to do certain actions (IE: You might need to run the workflow under the "App Step"