Viktor,
It is the server side OnSelectedDateChanged event that is not firing properly and/or if the DateInput is marked as disabled it will not retain the selected date. If I mark the DateInput controls as ReadOnly and remove the Enabled="false" attribute it does work but previously when I just had the Enabled="false" set without the ReadOnly attribute it worked fine and the server-side event fired just fine. I suppose I can live with the ReadOnly attribute but would like to know if the behavior was changed thus causing the server side event to not fire. There are no JavaScript errors being triggered so not sure what the issue is.
<telerik:RadDatePicker ID="LastDayRadDatePicker" runat="server" AutoPostBack="true" TabIndex="6" Width="100px" CausesValidation="false"
OnSelectedDateChanged="LastDayRadDatePicker_SelectedDateChanged">
<DateInput runat="server" Enabled="false" DateFormat="MM/dd/yyyy"></DateInput>
</telerik:RadDatePicker>