hi... i have  this problem too. when use radSchedule in AdvancedForm , Validators work but error message donot show..... but  when use firebug... i saw error message that Css causes error message hidden....
Help 

http://www.telerik.com/forums/validation-in-radscheduler-is-working-but-error-message-is-missing

How can this bug be classed as "New" when it's been around since 2013? For such an apparent and fairly major bug I can't believe it's been around so long, especially since the last couple of Telerik releases have revolved around stability and removing bugs?

I am also getting the same behavior in 2015.1.225.45 using Web Service bindings.

<telerik:RadScheduler runat="server" ID="uxRadScheduler_RepChatSchedule" HoursPanelTimeFormat="t" Height="600px"
                                    EnableExactTimeRendering="true" ShowAllDayRow="true" ShowFooter="true" SelectedView="WeekView" AppointmentStyleMode="Default"
                                    StartInsertingInAdvancedForm="true" StartEditingInAdvancedForm="true" 
                                    OnClientFormCreated="clientFormCreated" OnClientAppointmentsPopulating="appointmentsPopulating">
                                    <AdvancedForm Modal="true"></AdvancedForm>
                                    <TimelineView UserSelectable="false" ></TimelineView>
                                    <TimeSlotContextMenuSettings EnableDefault="true"></TimeSlotContextMenuSettings>
                                    <AppointmentContextMenuSettings EnableDefault="true"></AppointmentContextMenuSettings>
                                    <WebServiceSettings  Path="../../../../LocalWebServices/CustomerChatRepSchedulerService.asmx" ResourcePopulationMode="Manual"></WebServiceSettings>
                                </telerik:RadScheduler>