The error in question is the result of a conflict with the browser security rules.

Nowadays browsers have tightened their security and you cannot seamlessly load web pages in an iframe like https://www.google.com and https://www.telerik.com because this leads to an 'X-Frame-Options'  error.

For example, try placing this tag on any random aspx.html page

<iframe src="https:/www.google.com"></iframe>

this error will be thrown in the console:

Refused to display 'https://www.google.com/' in a frame because it set 'X-Frame-Options' to 'sameorigin'.

More details about the X-Frame-Option setup can be found at https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options .

In short, this setting specifies whether or not a browser should be allowed to render a page in a <frame>, <iframe>, <embed>, or <object>.

If you manage to overcome this browser limitation with an iframe, you will be able to achieve it with RadWindow too. When RadWindow is configured to use its NavigateURL property it renders the page in an iframe and this scenario is the same as adding an iframe on the page and setting the src to the URL you have.

If you get the same errors, this is a default browser behavior due to its security. You will need to research your site setup and update the frame settings for the site you are loading.

Kind regards,
Doncho
Progress Telerik