Thank you for contacting us with this issue. We investigated the problem, and it seems that there is a problem with the design-time serialization. It will be fixed for the next releases.
As a workaround you can set the value by code:
|public Form1() |
| InitializeComponent(); |
| radChart1.PlotArea.EmptySeriesMessage.TextBlock.Text = "my custom text"; |
the Telerik team