This file can help by debugging "user cannot log in" problems when in EPiServer the user is just redirected back to the login page when logging in. Probably you are authenticated but not authorized.

LoginTest.aspx

<%@ Page Language="C#" %>
<script runat="server">
  protected override void OnInit(EventArgs e){
    Response.Write(User.Identity.Name);
    Response.Write("<br>");
    foreach(var role in System.Web.Security.Roles.GetRolesForUser()){
      Response.Write(role);
      Response.Write("<br>");
    }
  }
</script>