.NET login control inconsistent

Question

I have the following login control using .net membership:

 <asp:Login ID="l_Main" runat="server" MembershipProvider="SqlProvider" DestinationPageUrl="Pages.aspx" />

Config:

<authentication mode="Forms" >
      <forms path="/ad" loginUrl="/Admin/Login.aspx"
        name=".ASPXFORMSAUTH" />
    </authentication>
    <membership defaultProvider="SqlProvider" userIsOnlineTimeWindow="15">
      <providers>
        <clear />
        <add
          name="SqlProvider"
          type="System.Web.Security.SqlMembershipProvider"
          connectionStringName="memString"
          applicationName="app1"
          enablePasswordRetrieval="false"
          enablePasswordReset="true"
          requiresQuestionAndAnswer="true"
          requiresUniqueEmail="true"
          passwordFormat="Hashed" />
      </providers>
    </membership>
    <roleManager enabled="true" defaultProvider="SqlRoleProvider">
      <providers>
        <clear/>
        <add name="SqlRoleProvider" applicationName="app1" connectionStringName="memString"
             type="System.Web.Security.SqlRoleProvider"/>
      </providers>
    </roleManager>

When i try to login from my computer it works as expected. When i have other users try to login they randomly are not able to login. One workaround I have discovered for them is to clear out everything after ".../Login.aspx" in the url, press enter, then press ctrl+F5. I assume that there is some sort of cookie problem but i'm not sure what. Do you see anything in the way i have this setup that might cause this problem?