Symptoms

The environment was recently upgraded from version 4.6.9 to 4.6.11. After the upgrade it was noticed that everything appears to be working, but there is a particular SmartObject event in the workflow that isn't properly firing off, sample error message below.

SmartObject out of sync - GetK2ApplicationSettings.... A new version of K2 for SharePoint may be required to make changes to the list/library..
 

Diagnoses

We checked the associated SO in the service tester and they all executed accordingly. Also confirmed site and apps updated properly after the upgrade. Ran a test solution with SO, deployed and did so successfully. Went back into original workflow, deleted error event, re-added and deployed. Solution is now working properly.

Resolution
We ended up making a copy of workflow solution that was throwing SO errors in K2 Studio. We deleted SO events and recreated with same mapping as before. Redeploying the workflow fixed the issue.