This error can be thrown if the physical path is used as a parameter to MapPath() method. However, in the source code of the content provider from the linked KB article, MapPath() is used only to locate the mapping file.
string physicalPathToConfigFile = Context.Server.MapPath(this.pathToConfigFile);
is a private filed in the class with the virtual path as value
private string pathToConfigFile = "~/App_Code/MappingFile.mapping"
Have you modified the value of pathToConfigFile? If so, please make sure that it is virtual path.
If this is not the case, could you please provide the exact configuration of RadFileExplorer, the MappingFile and the complete error's stack trace.
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