Fixing - System.Net.WebException: The remote server returned an error: (500) Syntax error, command unrecognized

I created FTP code to transfer files. This code works fine except that it sometimes causes an error 500. The exact error is -

Error:System.Reflection.TargetInvocationException:Exception has 
been thrown by the target of an invocation.--->System.Net.WebException:The remote server returned an error:(500)Syntax error, command unrecognized.
   at System.Net.FtpWebRequest.CheckError()
   at System.Net.FtpWebRequest.SyncRequestCallback(Object obj)
   at System.Net.CommandStream.Abort(Exception e)
   at System.Net.FtpWebRequest.FinishRequestStage(RequestStage stage)
   at System.Net.FtpWebRequest.GetRequestStream()
   at ST_772dn22cj49ndfddatee.csproj.ScriptMain.Main()---End of inner exception stack trace ---

I noticed that the error occurs when the biggest file is loaded, ie about 290 KB. All other files are less than this and i get no exception for them. I don't know why this happens. Can someone tell me why ?

As an aside, in case you notice some room for improvement in my code or logical error, then please mention that as well. I am not really looking for code reviews, but its welcome.

publicvoidMain(){Boolean conditions =true;if(conditions ==true){string fileLocation ="my windows directory";string fileName ="fileName.extension";string ftpFolder =@"/ftpFolder/";Boolean ftpMode =true;//passive or active. True = passive string ftpPassword ="password";int ftpPort =21;// the defaultstring ftpServerName ="server name";string ftpUserName ="user name";//Create an object to communicate with the server.string ftpRequestString ="ftp://"+ ftpServerName +":"+ ftpPort + ftpFolder + fileName;try{FtpWebRequest request =(FtpWebRequest)WebRequest.Create(ftpRequestString);
        request.Method=WebRequestMethods.Ftp.UploadFile;

        request.Credentials=newNetworkCredential(ftpUserName, ftpPassword);//Set modeif(ftpMode ==true){
            request.UsePassive=true;}//Copy the file to the request.string filePath =@fileLocation+"\\"+ fileName;StreamReader sourceStream =newStreamReader(filePath);byte[] fileContents =Encoding.UTF8.GetBytes(sourceStream.ReadToEnd());
        sourceStream.Close();
        request.ContentLength= fileContents.Length;Stream requestStream = request.GetRequestStream();
        requestStream.Write(fileContents,0, fileContents.Length);
        requestStream.Close();FtpWebResponse response =(FtpWebResponse)request.GetResponse();

        response.Close();}catch(WebException ex){MessageBox.Show(ex.Message);}//try-catch}}//main