Does the page with the RadWindow exists in the site's root? If so, this problem was reported several times during the years and our investigation shows that the problem is not directly related to our control, but to the way the framework works. Basically, our control does not affect the URL in any way - we leave this entirely to the framework's URL resolving logic.
There seems to be a problem with Request.ApplicationPath in some specific scenarios - check this blog post
for more details. As you will see, the problem occurs with the standard controls as well.
This being said, I would suggest to set the IconUrl property from code-behind, by using the ResolveUrl method, e.g.:
AddPersonNoteWindow.IconUrl = ResolveUrl(
I hope this helps. If it doesn't, or your scenario is different than the one described in the blog post, please provide more details about your exact setup so we can investigate further.
the Telerik team
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