This error usually displayed when Active directory is choosen to get ASP.net based website members
Just add : enableSearchMethods=”true” in the web.config file as detailed below

<membership defaultProvider="MyADMembershipProvider" >
      <providers>
        <add name="MyADMembershipProvider" enableSearchMethods="true"
        type="System.Web.Security.ActiveDirectoryMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, &#xD;&#xA;        PublicKeyToken=b03f5f7f11d50a3a"
        connectionStringName="ADConnectionString"
        attributeMapUsername="sAMAccountName"
        connectionUsername="Domain\UserName"
        connectionPassword="PassWord"/>
      </providers>
    </membership>

Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: 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.

Source Error:

Line 50:             ASP.NET to identify an incoming user. 
Line 51:         -->
Line 52:         <authentication mode="Windows" />
Line 53:         <!--
Line 54:             The <customErrors> section enables configuration

Source File: c:\inetpub\wwwroot\website1\web.config Line: 52
Source File: c:\inetpub\wwwroot\somewhere\web.config Line: 51
Cause

This error is generated because the default set of configuration values in web.config require IIS to treat the web site as an application but this has not been configured.

Solution:
1.Control Panel > Administrative Tools >IIS
2. Open webiste properties (Default Web Site>Your Website >Right click > Properties)
3. If the “application name” (under “Application Settings” on the “Directory” tab) is greyed out then click ‘[Create]‘ and then ‘[OK]‘.