This error means that the ASP.NET user (Network Service) cannot find the ConfigFile.xml or ListConfigFile.xml. These files are located by default at:
\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\Editor\.
If the files are not there, then you might have a corrupt installation and I suggest that you retract the editor solution and try to deploy it again.
If the files are there, then you have a problem with file permissions. Make sure that the ASP.NET user has at least read rights on the editor folder.
All the best,
the Telerik team