nhibernate sql Express connection issue - error: 26 - Error Locating Server/Instance Specified

I can connect fine with normal ado.net. However i get the following error when i tried to connect nHibernate.

## hibernate.cfg.xml
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
  <session-factory>
    <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>
    <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
    <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
    <property name="connection.connection_string">Server=xxxxx\SQLEXPRESS; Database=xxxxx; User ID=xxxxx; Password=xxxxx; Trusted_Connection=True</property>    
    <property name="proxyfactory.factory_class">NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle</property>
    <property name="show_sql">true</property>    
  </session-factory>
</hibernate-configuration>

## Server error
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

## Full stack

[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)]
   System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) +4845255
   System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) +194
   System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, SqlConnection owningObject) +4858557
   System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject) +90
   System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(String host, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart) +342
   System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance) +221
   System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance) +189
   System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection) +185
   System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options) +31
   System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject) +433
   System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject) +66
   System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) +499
   System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) +65
   System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +117
   System.Data.SqlClient.SqlConnection.Open() +122
   NHibernate.Connection.DriverConnectionProvider.GetConnection() +102
   NHibernate.Tool.hbm2ddl.SuppliedConnectionProviderConnectionHelper.Prepare() +15
   NHibernate.Tool.hbm2ddl.SchemaMetadataUpdater.GetReservedWords(Dialect dialect, IConnectionHelper connectionHelper) +65
   NHibernate.Tool.hbm2ddl.SchemaMetadataUpdater.Update(ISessionFactory sessionFactory) +80
   NHibernate.Impl.SessionFactoryImpl..ctor(Configuration cfg, IMapping mapping, Settings settings, EventListeners listeners) +599
   NHibernate.Cfg.Configuration.BuildSessionFactory() +87
   XXX.Domain.Repositories.NHibernateHelper.get_SessionFactory() in D:\dev\MyProject\XXX\XXX.Domain\Repositories\NHibernateHelper.cs:23
   XXX.Domain.Repositories.NHibernateHelper.OpenSession() in D:\dev\MyProject\XXX\XXX.Domain\Repositories\NHibernateHelper.cs:31
   XXX.Domain.Repositories.EntryRepository.GetCountByGmapId(Int32 gmapId) in D:\dev\MyProject\XXX\XXX.Domain\Repositories\EntryRepository.cs:152
   XXX.Controls.Activity.BindRepeater(Int32 id) in D:\dev\MyProject\XXX\XXX.Controls\Activity.ascx.cs:58
   XXX.Controls.Activity.DropDownListMaps_SelectedIndexChanged(Object sender, EventArgs e) in D:\dev\MyProject\XXX\XXX.Controls\Activity.ascx.cs:75
   System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e) +111
   System.Web.UI.WebControls.DropDownList.RaisePostDataChangedEvent() +134
   System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent() +10
   System.Web.UI.Page.RaiseChangedEvents() +165
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1485