Did you set the validationgroup property?

<asp:ValidationSummary ID="XFormValidationSummary" runat="server" ValidationGroup="XForm" />

I don't manage validation groups at all, but error messages from all another validators (CompareValidator for example) appear in top epi summary control but only from CustomValidator do not. Summary control appears with warning icon and empty string instead a real error  message.

Could you post the code for your XForm control? (the markup from xform.ascx)

It is not XForm. It's form setting for a dynamic content. Murckup and C# codу you can find below. Thanks for response.

<%@ Control Language="C#" CodeBehind="QuickPollSettings.ascx.cs" Inherits="Maggie.Web.CustomPropertyTypes.DynamicContent.QuickPoll.QuickPollSettings" %>
<%@ Import Namespace="EPiServer.Core" %>
<asp:ScriptManager runat="server" ID="scriptManager">
</asp:ScriptManager>
<div class="quickPollFilter">
 <asp:CustomValidator ID="dateRangeValidator" EnableClientScript="false" runat="server" OnServerValidate="dateValidate" Text="End date should be great than start date"/>
<p class="quickPollFilterTitle">
<%= LanguageManager.Instance.Translate("/quickPoll/selectFilterText")%>
</p>
<div class="epirowcontainer">
<label class="epiindent episize100">
<%= LanguageManager.Instance.Translate("/quickPoll/filterStartCreationDateText")%>
</label>
<EPiServer:InputDate ID="cdStartDayFilter" runat="server" style="display: inline;" DisplayName="Start Date" DisplayTime="false" ValidateInput="true" />
</div>
<div class="epirowcontainer">
<label class="epiindent episize100">
<%= LanguageManager.Instance.Translate("/quickPoll/filterEndCreationDateText")%>
</label>
<EPiServer:InputDate ID="cdEndDayFilter" runat="server" DisplayName="End Date" style="display: inline;" DisplayTime="false" ValidateInput="true"/>
</div>
<div class="epirowcontainer">
<span style="white-space:nowrap">
<label class="epiindent episize100">
<%= LanguageManager.Instance.Translate("/quickPoll/filterNameText")%>
</label>
<asp:TextBox runat="server" ID="tbFilterName" CssClass="episize240"></asp:TextBox>
<asp:Button runat="server" ID="btApplyFilter" OnClick="btApplyFilter_Click" Text='<%# LanguageManager.Instance.Translate("/quickPoll/applyButtonText")%>'>
</asp:Button>
</span>
</div>
<div class="epirowcontainer">
<label class="epiindent episize100">
<%= LanguageManager.Instance.Translate("/quickPoll/pollList")%>
</label><span class="quickPollListBox">
<asp:ListBox ID="lbPolls" DataTextField="Key" CssClass="episize240" DataValueField="Value" runat="server"
SelectionMode="Single" Rows="1" />
</span>
</div>
</div>

Code behind C#

protected void dateValidate(object source, ServerValidateEventArgs args)
{
if (cdEndDayFilter.Input.Text.Length > 0 && cdEndDayFilter.Value < cdStartDayFilter.Value)
{
args.IsValid = false;
}
}