Symptoms
Error when open the runtime forms:-
"Unable to find assembly 'SourceCode.Forms.Runtime.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=16a2c5aaaa1b130d'."

Diagnoses
This error is persistent in all forms.

Resolution
This is due to missing assembly mentioned in nProgramFiles]K2 BlackpearlHostServerBin and iProgramFiles]K2 BlackpearlBin folder. Based on past experiences, the file wouldn't went missing without a reason and it's normally due to deployment, software update and etc. Unfortunately there is no way to tell exactly why the file went missing. This could be due to recent deployment of a coldfix to production environment. Resolved after put back the missing assembly "SourceCode.Forms.Runtime.Common" into respective folders.