asp.net - Required Field Validator only working in IE - not in chrome or firefox

Question

In my asp.net (vb on server side) application I am using a required field validator when a submit button is hit. The validator checks the value of txtLatestEnd - which is a hidden text box. The value of this text box is changed via javascript when the user selects an item on the screen before hitting submit.

I have noticed that the required field validator is only working in IE, but not chrome or firefox. Could anyone see why this is happening?

Required field validator

<asp:RequiredFieldValidator ID="reqLinks" Runat="server" ControlToValidate="txtLatestEnd" Display="Dynamic"
EnableClientScript="true" ErrorMessage="Links are required" CssClass="detailrow" Font-Bold="true"></asp:RequiredFieldValidator>

Edit

If it helps - here is the function on the server side that handles the button click

Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSave.Click
    UpdateDatabase()
End Sub

and here is the button itself.

<asp:Button Runat="server" ID="btnSave" Text="Save Milestone" CssClass="smalltextbutton" CausesValidation="True"></asp:Button>

Both the validator and button are their own table rows in an asp.net table

Edit 2: as requested, here's the code for the "hidden" field - by hidden, it is inside a div that is not displayed

<div id="divHiddenFields" style="DISPLAY:none">
    ---------------Hidden Fields---------------------------------------------------<br>
    Latest end date:<input type="text" ID="txtLatestEnd" Runat="server"><br>
    IDs &amp; Cats<input type="text" style="WIDTH:50%" runat="server" id="IDCATList"><br>
    creator: <input type="text" runat="server" id="hdnCreator"><br>
    county county<input type="text" runat="server" id="hdnLinkCount" value="0"><br>
    preloadDateArray<input type="text" runat="server" id="hdnPreLoadDates">
</div>

I have also tried moving the button and validator outside of the table, this does nothing to make the validator work.