Your issue is caused by the fact that the callback does not update the whole page. For example, during a callback to the ASPxGridView only the GridView content is updated. As ASPxCallback is not a visible control, the page cannot be updated during its callback. Please refer to the K18387 KB article for more information.
You can use the CallbackEventArgs.Result property to pass some text from the server to the client using the ASPxCallback control. You can handle the client-side ASPxClientCallback.CallbackComplete event to get this string. Pass your error message to the client using this parameter and then use the client-side functionality to assign this message to your label.
If you wish to display a text in ASPxLabel, I recommend you initialize the ClientInstanceName property and use its value on the client side to get the control client-side instance. After that, call the ASPxClientLabel.SetText method to set the text.
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