ETags are GUIDs. In your case the Guid seems not to be generated or it is not valid due to some configuration( IIS , load balancer or some values for the ASP.NET caching) . It seems that you use SlidingExpiration and ASP.NET caching for pages where SetETagFromFileDependencies is called. This method creates an ETag by combining the file names and last modified timestamps for all files on which the handler is dependent. ETag values are unique identifiers that are generated by the server and changed every time the object is modified.
All the best,
the Telerik team
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