You are right @Seyed Morteza Mousavi, my problem was in the register link!
The default looked like this when I created the site.
<asp:HyperLink runat="server" ID="RegisterHyperLink" ViewStateMode="Disabled">Register</asp:HyperLink> if you don't have an account.
I changed it to be this.
<asp:LinkButton runat="server" ID="lnkRegister" PostBackUrl="~/Account/Register.aspx" CausesValidation="false">Register</asp:LinkButton> if you don't have an account.
It works now. I don't know why default one doesn't work but I don't care anymore. Also, the Register link in the Site.Master next to Login link does not work. It keeps redirecting to Login page! Again not sure why. But this is OK for now.