Interop Word Document Class is null on Windows Server 2008 - Word Open Method

As long as I open the document and save it on my machine, it works fine, but when I upload it to the server and open it there, it goes to the block if (doc == null)

, it shouldn't go.

Please update my question title if it is irrelevant or asks for clarification.

Here is my class:

using System;
using System.Collections.Generic;
using System.Web;
using Microsoft.Office.Interop.Word;

/// <summary>
/// Summary description for ClsWordExManager
/// </summary>
public class ClsWordExManager
{
     public enum Extension
        {
            WebPage = 0
        }

        private static string HtmExtension
        {
            get
            {
                return ".htm";
            }
        }

        private static Application objWordApp = null;
        private static object objMissing = System.Reflection.Missing.Value;
        private static Document doc = null;
        static ClsWordExManager()
        {
            try
            {
                objWordApp = new Application();
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        public static void InitializeClass()
        {
            objWordApp.Visible = false;
        }

        private static string Open(object strFilePath)
        {
            string str = string.Empty;
            try
            {
                objWordApp.Visible = false;
                str += "<br /> word App visiblitly false";
            }
            catch (Exception ex)
            {
                objWordApp = new Application();
                str += ex.Message;
            }
            try
            {
                doc = objWordApp.Documents.Open(ref strFilePath, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing);
                str += "<br /> word document opened";
                if (doc == null)
                {
                    // It is null when I upload it on Windows Server 2008 with office 2007 installed.
                    str += "<br /> After openging its null";
                }
            }
            catch (Exception ex)
            {
                Close();
                objWordApp.Visible = false;
                str += "<br /> word document closed with : " + ex.Message;
            }
            return str;
        }

        private static void Close()
        {
            try
            {
                doc.Close(ref objMissing, ref objMissing, ref objMissing);
            }
            catch
            {
            }
        }


        private static string SaveAs(string FilePath, string strFileExtension, WdSaveFormat objSaveFormat)
        {
            try
            {
                if (ClsCommon.IsValidUser()) // impersonating User
                {
                    FilePath = System.IO.Path.ChangeExtension(FilePath, strFileExtension);
                    try
                    {
                        if (doc != null)
                        {
                            object objFilePath = FilePath;
                            object objFormat = objSaveFormat;
                            doc.SaveAs(ref objFilePath, ref objFormat, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing, ref objMissing);
                        }
                        else
                        {
                            FilePath += "document value is null";
                        }
                    }
                    catch
                    {
                        FilePath += "<br /> Saving document throwing expe";
                        return FilePath;
                    }

                }
                else
                {
                    FilePath += "<br /> Not valid for saving file ";
                }
            }
            catch (Exception ex)
            {
                FilePath += ex.Message;
            }
            finally
            {
                Close();
            }
            return FilePath;
        }

        public static string ReadWordFile(string strFilePath, Extension objExtension)
        {
            string strFileContent = "<br /> Reading Word File could not be completed";
            try
            {
                strFileContent += Open(strFilePath);
                if (objExtension == Extension.WebPage)
                {
                    try
                    {
                        string strNewFileName = SaveAs(strFilePath, HtmExtension, WdSaveFormat.wdFormatFilteredHTML);
                        if (strNewFileName != "")
                        {
                            strFileContent += strNewFileName + ClsCommon.ReadFile(strNewFileName, true); // ignore this line as it just read html file.
                        }
                        else
                        {
                            strFileContent += "file not saved";
                        }
                    }
                    catch (Exception ex)
                    {
                        strFileContent += ex.Message;
                    }
                }
                else
                {
                    Close();
                }
            }
            catch (Exception exx)
            {
                strFileContent += exx.Message;
            }
            return strFileContent;
        }

        public static void Quit()
        {
            try
            {
                objWordApp.Quit(ref objMissing, ref objMissing, ref objMissing);
            }
            catch
            {

            }
        }
}