When downloading a file, the Response needs to be cleared and populated with the file data which will be returned to the client.
When an AJAX request is triggered, it expects a given format of the response and that is why the download does not work in this scenario.
The solution is to ensure that the postback that is triggered when a file should be downloaded is a full one, with AJAX disabled. You can, for example, disable AJAX via the client-side OnRequestStart Event
based on the control that triggers the postback.
Also, keep in mind that the content-type is not the same for the xlsx, csv and xls file types: https://filext.com/faq/office_mime_types.html
If the issue persists, please provide a simple runnable project with dummy data and the same AJAX configuration that replicates the issue and attach it to an official support ticket. That would allow us to investigate locally and help you more efficiently.
Once we have a solution in the support ticket, we can share it here for convenience and better visibility from the community.
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.