From the provided description I cannot understand how this is related to the RadEditor control. The control is not related directly to the IIS and this does not lead me to any possible reasons for the described behavior.
If the configuration of the RadEditor control has not been changed or the version is not upgraded you should track any possible changes that could lead to the described problem.
If you are certain that the RadEditor control is related, please provide more details (e.g. How it appears broken? How it is related to the IIS reset? What changes has been done regarding the control?)
In case you desire to upgrade the used Telerik UI for ASP.NET AJAX suite, I recommend using the latest available one.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.