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This error might occur if the User data files which are used by the Telerik Control Panel are somehow corrupted. In order to make sure that the User data files are not corrupted and causing the issue I would suggest performing the following steps:
- Open a Windows Explorer window and navigate to %appdata%\Telerik\Installer\UserSettings
- Delete the content of the folder (LoginData, NotificationAuthenticationData and User.data files)
After these steps, please try to run the Telerik Control Panel again and try to apply your settings.
Please let me know if the issue still occurs.