Implementing the Application Initialization for IIS 7.5 (Server 2008R2) in 4.6.6, can cause the SmartForms sites (Designer and Runtime) to break after the upgrade to 4.6.11.
An "Unable to bring up Designer or Runtime, HTTP 500 error" error appears.
Server Error in Application "K2/Designer"
'HTTP Error 500.0 - Server Error
The page cannot be displayed because an internal server error has occurred'
The "AppWarmupModule" was registered in Modules in IIS. When attempting to load SmartForms sites after the upgrade, IIS first will load this module.
Since the module has not yet configured, it stops K2 Designer/Runtime site to load correctly, see IIS forum link for more details:
Go to "K2 sites" in IIS and delete the module: i.e. AppWarmupModule from IIS > SmartForms Site > modules > and delete AppWarmModule.
The SmartForms sites will render correctly.