Forms Login Fails In Windows Domain?

I'm currently integrating a web application into a customers network. The application has been successfully used many times.But here we've got the following problem:If the login page is requested just by the hostname everything works fine, but if the FQDN is used the login page just reloads withous doing anything. There's no error displayed.http://hostname:port/Login.aspx => fine

http://hostname.intern.customer.domain.foo.com => doesn't work, no error
(The FQDN is quite long)

The login page is very simple:

<asp:Login ID="loginCtrl" runat="server">
<LabelStyle HorizontalAlign="Left" />
</asp:Login>

Codebehind:

protected override void OnLoad(EventArgs e)
{
if (!this.IsPostBack)

[code]...