Symptoms

With no apparent reason users are getting below exception while using runtime URL from one of the K2 server within the farm.

Server Error
Exception of type 'System.Web.HttpUnhandledException' was thrown.
Extender control '7d2fb727-59ae-6fff-41a0-5cdab6e63109_85fb58be-d668-5767-446f-f75d637c675c' is not a registered extender control. Extender controls must be registered using RegisterExtenderControl() before calling RegisterScriptDescriptors().
Parameter name: extenderControl

Diagnoses

This is known issue which occurs in very rare circumstances and normally can be resolved by means of IIS reset or, using better approach, by recycling K2 smartforms runtime application pool.
 

Resolution

As a quick workaround perform recycle of  K2 smartforms runtime application pool. In case it is possible for you to maintain environment in such state and investigation of root cause is required - please log a ticket with K2 support.