ASP.NET validator never validates as true - asp.net

Question

The code below has always worked, but now somehow I can't submit the form anymore even though I provide both username and password. I get no errors in backend or the Chrome Developer Console. I tried enabling/disabling all validators and found that if I add Enabled="false" to validator UserNameRequired, the form does submit. But obviously I want to have this requiredfieldvalidator enabled.
Why is this happening?
You can also see it live here (fill in a random email address/password): http://www.zorgbeurs.nl/login
<table width="300">
<tr>
<td>
<asp:Label ID="UserNameLabel" Text="Email" runat="server" AssociatedControlID="UserName"/>
</td>
<td>
<asp:TextBox ID="UserName" Text="" runat="server" Width="160px"/><br />
<asp:RequiredFieldValidator Enabled="false" ValidationGroup="loginuser" Display="Dynamic" SetFocusOnError="True" ID="UserNameRequired" runat="server" ControlToValidate="UserName" ErrorMessage="*"/>
<asp:RegularExpressionValidator ControlToValidate="UserName" ValidationGroup="loginuser" ErrorMessage="Ongeldig emailadres" ValidationExpression="<%$resources:glossary,regexValidEmail %>" ID="revUsername" runat="server" Display="Dynamic"/>
</td>
</tr>
<tr>
<td valign="top">
<asp:Label ID="PasswordLabel" Text="<%$ Resources:Glossary,Password%>" runat="server" AssociatedControlID="Password"/>
</td>
<td valign="top">
<asp:TextBox ID="Password" runat="server" TextMode="Password" Width="160px"/><br />
<asp:RequiredFieldValidator Enabled="false" SetFocusOnError="True" Display="Dynamic" ValidationGroup="loginuser" ID="PasswordRequired" runat="server" ControlToValidate="Password" ErrorMessage="*" />
</td>
</tr>
<tr>
<td>
</td>
<td>
<asp:ImageButton CausesValidation="true" ValidationGroup="loginuser" ID="LoginMember" meta:resourcekey="ibtnLogin" CommandName="Login" runat="server"/>
<asp:CheckBox TextAlign="Right" CssClass="cblist" Font-Bold="false" ID="RememberMe" runat="server" Text="<%$ Resources:Glossary,loginrememberme%>" /><br />
</td>
</tr>
<tr>
<td colspan="2">
<asp:Label ID="FailureText" runat="server" ForeColor="Red" Font-Bold="true" EnableViewState="False"/>
</td>
</tr>
</table>
update
I also tried adding the ValidationExpression of revUsername expression directly in the code [-+.'\w]+#[-.\w]+\.[-.\w]+, but that does not help.

Answer 1

I can see the required validators working in your live site without making a postback.
Regarding the email's regular expression validator, it seems that your expression is not working properly. So, I suggest you to change your regular expression, try this one:
<asp:RegularExpressionValidator Enabled="true" ControlToValidate="UserName"
ValidationGroup="loginuser" ErrorMessage="Ongeldig emailadres"
ValidationExpression="\w+([-+.']\w+)*#\w+([-.]\w+)*\.\w+([-.]\w+)*"
ID="revUsername" runat="server" Display="Dynamic" />
Also, remember to enable it again in order to make it work both on client and server side.
Hope it can help you!