Symptoms

After upgrading K2 to 4.6.11 and then opening a Form using the secondary / additional Runtime site (called RuntimeAD in IIS), the following error appears in the web browser:

System.Web.HttpUnhandledException
The server is not licensed
 

Diagnoses

It seems the client components for the secondary / additional runtime site have not been updated.
 

Resolution

The alternate Runtime Site called "RuntimeAD" had it's folder located in the K2 Blackpearl directory.

When K2 was upgraded, it failed to upgrade this folder as well. There were also custom controls installed in this alternative runtime site.

The solution to this was to delete and recreate the RuntimeAD alternative runtime site again in IIS and do the same with the folder inside the K2 Blackpearl directory, making sure it contained the same components of the default Runtime site plus the custom controls.