Symptoms

One application fails to work after upgrade from 4.6.8 to 4.12060.1731.2 (4.6.11)
 

Diagnoses

Installed 4.12060.1731.2 on DEV machine - web app working fine, Installed 4.12060.1731.2 on PRD machine - web app is not working properly on PRD all K2 components are the same version, error in web app is: Method could not be resolved! What could be the cause of this issue?
 

Resolution

a) on DEV the app and K2 was installed on the same server so the app used DLLs from windows assembly (new ones)

b) on PRD app was installed on separate server and used old DLLs from 2013 and 2014.

So to get around this you need to include the new DLLs in the app's bin folder after this everything should start to work as expected.