Symptoms

"SmartObject property K2_Int_LookupDisplayColumnName is a required property for selected method GetListItemsForLookup." with lookup control.
 

Diagnoses

Issue: "SmartObject property K2_Int_LookupDisplayColumnName is a required property for selected method GetListItemsForLookup." with lookup control.
 

Resolution

Resolution: The lookup column in SharePoint was a renamed 'Title' column. For some reason SharePoint and/or K2 holds onto a reference of the columns original name. The rule that calls Get List Items For Lookup must have been passing in an invalid or null name in this case it should be 'Title' instead of the current name. In troubleshooting, we deleted the control, re-added the rule, and included 'Title' as the method's input and it worked.