It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.
<system.web>
<compilation debug="true" targetFramework="4.0">
<assemblies>
<add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
</assemblies>
</compilation>
</system.web>
<location path="Login.aspx">
<system.web>
<customErrors mode="Off" defaultRedirect="~/Login.aspx">
</customErrors>
<authentication mode="Windows">
</authentication>
</System.web>