Symptoms

We are unable to use K2 for SP on any site. Getting the following error when clicking on K2 application from SharePoint ribbon: Thread 1: Object reference not set to an instance of an object. Have no issue running activation for the site.
 

Diagnoses

The issue was caused by a change to the value of SmartForms Designer URL and SmartForms Runtime URL in environment library. The change causes the value of these fields in K2 Setting hidden list to not match the environment fields.
/SP Site]/Lists/K2%20Settings/AllItems.aspx VS Environment Fields

 

Resolution

Changing the values of these fields in Environment Fields to match the K2 Setting hidden list resolved the issue.