We have received similar reports only for charts used in a load balanced environment and we suppose that is the case with your project as well -- when RadChart is placed in a load balanced environment, there could be an issue if the page is served and therefore the chart image is generated by one machine and the next request for the image itself is served by another machine. Then this second machine will not be able to find the image in the session. The solution would be to implement State server or SQL server session state mode
or sticky sessions, ensuring that all the requests from one user will be handled by the same server.
Specifically for the popup -- it is only visible if the project is run in Debug mode; if you are running in Release mode the chart will not display in the described scenario but you will get no popup either.
Hope this helps.
All the best,
the Telerik team