Unable to establish secure connection with the server

I have a website that has been using AD to authenticate but it is no longer working and I get the error;

Unable to establish secure connection with the server

My web config settings are;

 <connectionStrings>
      <add name="ADConnectionString" connectionString="LDAP://xxx.com/CN=Users,DC=x,DC=x,DC=com"/>
 </connectionStrings>

  <membership defaultProvider="MyADMembershipProvider">
   <providers>
    <add name="MyADMembershipProvider"
            type="System.Web.Security.ActiveDirectoryMembershipProvider,
            System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
      &nb ...