I'm assuming PreProd is a copy of dev/UAT and not something you built from scratch or changed after the fact, right?
I'm still working strictly in dev and getting this error. If there's a chance a different environment wouldn't have the issue, I can plan to do a trial migration early and see if that resolves it.
Did you already check the requirements mentioned in this document on page 9: LazyApproval Guide: Nintex Workflow 2013 & Nintex Workflow 2010
On our site it could be caused by the fact that we are running the email service not on a frontend. We are going to change but as it is production it is not that easy to do this during the work day.
This error typically indicates a Topology configuration issue.
For LazyApproval to work, it will need to have the following three services on the same server that will process the lazy approval:
1: The “Microsoft SharePoint Foundation Web Application”.
2: The "Microsoft SharePoint Foundation Workflow Timer Service".
3: The SMTP service enabled.
If you review your environment you may find that the server you have SMTP on is not actually running the Web Application Service. Please be aware that these are also recommendations from Microsoft as well in their incoming email configuration guide here: Configure incoming email for a SharePoint 2013 farm
"each SharePoint front-end web server must be running the Simple Mail Transfer Protocol (SMTP) service and the SharePoint Foundation Web Application service."
Please be aware that there are advanced configuration options as well that allow for additional adjustments to the SMTP service but the primary point from a Nintex perspective is that the SharePoint server that is receiving the lazy approval emails has our solutions deployed to it (meaning it is a web front end server).
Update: please note this topic no longer applies to the latest release of Nintex Workflow for 2010 (build: 188.8.131.52) and 2013 (build: 184.108.40.206) and later as these latest builds are deployed Globally rather than per Web Application.
Thank you for this advice. This turned out to be the solution for me.
Both the Web Application and Workflow Timer services were stopped.
When we initially tried to start the Web Application service, it still showed a status of “Starting” the next day. We had to do some research and run the following commands with a privileged account to get it unstuck:
Once we did that, the service started and our next Lazy Approval test worked.