RadChart stores its image in the cache for 5 minutes. If the image is requested during these 5 minutes, the timing is restarted, i.e. the image will be stored in the cache for another 5 minutes. It seems that the image has been requested after these 5 minutes have been elapsed. However, if the page with the chart has been requested, a new image should have been generated, so this error should not appear.
How often does this error occur? Can you provide more details about your scenario? Do you have a page with RadChart and another page which points to the chart in the first one by any chance?
the Telerik team