friendly URLS Login page postback - asp.net

Question

Problem
I'm working on the loginscreen of my website. I used my own template but added the "RequiredFieldValidator" for my password and username. When I click the ASP:Button without filling in the textboxes. The RequiredFieldValidator errormessages appears for 1 second but then the page refreshes.
Platforms
Firefox
Chrome
Opera
My thoughts
I think this has to do with the friendly urls nuget. I didn't touch this but when I click the login button and look at the networkevents (in Chrome - F12) I see that it's /Account/Login?.......... so without the '.aspx'
HTML-Code
<form role="form">
<div class="form-group">
<div class="input-group login-input">
<span class="input-group-addon"><i class="fa fa-user"></i></span>
<asp:TextBox runat="server" id="LoginUserName" type="text" class="form-control" placeholder="Username"/>
<asp:RequiredFieldValidator runat="server" ControlToValidate="LoginUserName"
CssClass="text-danger" Display="Dynamic" ErrorMessage="The user name field is required." />
</div>
<br>
<div class="input-group login-input">
<span class="input-group-addon"><i class="fa fa-lock"></i></span>
<asp:TextBox runat="server" id="LoginPassword" type="password" class="form-control" placeholder="Password"/>
<asp:RequiredFieldValidator runat="server" ControlToValidate="LoginPassword"
CssClass="text-danger" Display="Dynamic" ErrorMessage="The password field is required." />
</div>
<div class="checkbox">
<label>
<input runat="server" id="InputRememberMe" type="checkbox">
Remember me
</label>
</div>
<div class="input-group login-input">
<asp:Button runat="server" onclick="LogIn" class="btn btn-primary pull-right" Text="Login" />
<asp:PlaceHolder runat="server" ID="ErrorMessage" Visible="false">
<p class="text-danger">
<asp:Literal runat="server" ID="FailureText" />
</p>
</asp:PlaceHolder>
</div>
<hr>
Create Account
Password Recovery
<div class="clearfix"></div>
</div>
C# Code
protected void Page_Load(object sender, EventArgs e)
{
//RegisterHyperLink.NavigateUrl = "Register";
//OpenAuthLogin.ReturnUrl = Request.QueryString["ReturnUrl"];
//var returnUrl = HttpUtility.UrlEncode(Request.QueryString["ReturnUrl"]);
//if (!String.IsNullOrEmpty(returnUrl))
//{
// RegisterHyperLink.NavigateUrl += "?ReturnUrl=" + returnUrl;
//}
}
protected void LogIn(object sender, EventArgs e)
{
if (IsValid)
{
// Validate the user password
var manager = new UserManager();
ApplicationUser user = manager.Find(LoginUserName.Text, LoginPassword.Text);
if (user != null)
{
IdentityHelper.SignIn(manager, user, InputRememberMe.Checked);
IdentityHelper.RedirectToReturnUrl(Request.QueryString["ReturnUrl"], Response);
}
else
{
FailureText.Text = "Invalid username or password.";
ErrorMessage.Visible = true;
}
}
}

Answer 1

I solved my problem. I had a form tag in my masterpage but also in my login page which gives a warning. This blocked everything...