Symptoms

When clicking on K2 Application from SharePoint List/Library on the root site and 2 other sites for this site collection, the app will be stuck and displays "Working on it.."
 

Diagnoses

The issue was caused by old k2 instance of appit was set in the K2 Hidden list for these sites which can be navigated to using URL in the following format: https://pSite URL with issue]/lists/k2 settings
 

Resolution

After updating SmartForms_Runtime_URL on the hidden list for these site to use the correct value ,and perform "Activate Site Collection" via Site Content > K2 Appit for SharePoint > Activate Site Collection, the issue was resolved.