Symptoms

When creating a new workflow in SharePoint 2013, we saw the following error:
Exception: Sequence contains more that one matching element.
 

Diagnoses

Upon investigating, we found that the "SharePoint Integration Helper Methods" smartobject was missing the "Get Designer Data" method. We were able to see this using the smartobject tester tool and navigating to "SmartObject Explorer" > "System" > "SharePoint 2013 Integration" > "SharePoint Integration Helper Methods". ("Get Designer Data" was missing.)
 

Resolution

Using PandD, we deployed the "K2 for SharePoint 2013.kspx" package found under "rK2blackpearlInstallDir]K2 for SharePoint 2013 Setup" folder. After deploying the package again, we verified that the "Get Designer Data" method was back and we created a test workflow to test functionality and all was well.