Works everywhere else. This my code around it:
<div id="divddHealthConditions" class="inputWrapper marginRight">
<asp:Label ID="lblHealthConditions" runat="server" CssClass="lblddHealthConditions" Text="<%$Resources: Resources, strHealthConditions %>" AssociatedControlID="ddHealthConditions" />
<telerik:RadDropDownList ID="ddHealthConditions" runat="server" RenderMode="Lightweight" CssClass="ddHealthConditionsAttr" AutoPostBack="True" CausesValidation="False" DefaultMessage="<%$ Resources:Resources, strDefaultMessageDDLPleaseSelect %>" OnSelectedIndexChanged="ddHealthConditions_SelectedIndexChanged" OnClientSelectedIndexChanged="DdlGetSelectedIndexChanged"></telerik:RadDropDownList>
<div class="clearer">
<asp:CustomValidator ID="cvDdHealthConditions" runat="server" CssClass="validator" ValidationGroup="<%$Resources: Resources, strVgHealthConditions %>" ErrorMessage="<%$Resources: Resources, strErrMsgReqField %>"
ControlToValidate="ddHealthConditions" Display="Dynamic" Enabled="true" Visible="true" EnableClientScript="true">
</asp:CustomValidator>
</div>
</div>
<div id="divddHealthConditions" class="inputWrapper marginRight">
<asp:Label ID="lblHealthConditions" runat="server" CssClass="lblddHealthConditions" Text="<%$Resources: Resources, strHealthConditions %>" AssociatedControlID="ddHealthConditions" />
<telerik:RadDropDownList ID="ddHealthConditions" runat="server" RenderMode="Lightweight" CssClass="ddHealthConditionsAttr" AutoPostBack="True" CausesValidation="False" DefaultMessage="<%$ Resources:Resources, strDefaultMessageDDLPleaseSelect %>" OnSelectedIndexChanged="ddHealthConditions_SelectedIndexChanged" OnClientSelectedIndexChanged="DdlGetSelectedIndexChanged"></telerik:RadDropDownList>
<div class="clearer">
<asp:CustomValidator ID="cvDdHealthConditions" runat="server" CssClass="validator" ValidationGroup="<%$Resources: Resources, strVgHealthConditions %>" ErrorMessage="<%$Resources: Resources, strErrMsgReqField %>"
ControlToValidate="ddHealthConditions" Display="Dynamic" Enabled="true" Visible="true" EnableClientScript="true">
</asp:CustomValidator>
</div>
</div>