As I see from the screenshot provided - you use RadComboBox nested within RadWindow.
Is there an AJAX postback performed at your page?
Let me suggest you set the EnableAjaxSkinRendering property of the RadComboBox to "true".
If the issue persists - please explain your implementation in more details and provide us a simplified code that reproduces it.
All the best,
Kalina
the Telerik team
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