Error 2 Build failed due to validation errors in C:\RPT.dbml

I am getting the following error message in a C# 2008 application and I do not know what to look at to resolve the problem:
Error2Build failed due to validation errors in C:\RPT.dbml.  Open the file and resolve the issues in the Error List, then try rebuilding the project.

There is nothing showing me where the exact error is coming from. There is a reference setup for System.Configuration.

The following code is from the RPT.designer.cs


namespace cli
{
using System.Data.Linq;
using System.Data.Linq.Mapping;
using System.Data;
using System.Collections.Generic;
using System.Reflection;
using System.Linq;
using System.Linq.Expressions;
using System.ComponentModel;
using System;
    using System.Configuration;


[System.Data.Linq.Mapping.DatabaseAttribute(Name="DEV")]
public partial class RPTDataContext : System.Data.Linq.DataContext
{

private static System.Data.Linq.Mapping.MappingSource mappingSource = new AttributeMappingSource();

  

public RPTDataContext() :
        base(ConfigurationManager.ConnectionStrings["DEVConnectionString"].ConnectionString.ToString(), mappingSource)
{
OnCreated();
}

The following code is from RPT.cs file:

namespace cli
{
    partial class RPTDataContext
    {
    }
}

Here is the code in the app.config file:

          connectionString="Data Source=xxx;Initial Catalog=PROD1;Integrated Security=True"
      providerName="System.Data.SqlClient" />
 

Thus can you tell me and/or show me in code how to fix this problem?