Hi,
For anybody interested, I have spotted the issue.
The log IS written BUT guess where?
The name of the flat file sink was stated as "myfile.log" with no preceding
directory.
THe logging block opens creates file without  problem in the directory my
exe is located, and writes the entries until...
The windows app. uses an openfile dialog and after the second the openfile
dialog is used A NEW log file is created at the directory where the openfile
dialog pointed earlier. But I, having seen the first file created normally
open it and see no entries after openfile action. This is the user
perspective, the BUG lies in the WriteMessageToFile method of FlatFileSink
class :
       private void WriteMessageToFile(LogEntry logEntry)
       {
           FlatFileSinkData flatFileSinkData =
GetFlatFileSinkDataFromCursor();
           string directory =
Path.GetDirectoryName(flatFileSinkData.FileName);
           if (directory.Length == 0)
           {
               directory = AppDomain.CurrentDomain.BaseDirectory;
           }
           if (!Directory.Exists(directory))
           {
               Directory.CreateDirectory(directory);
           }
           using (FileStream fileStream = new
FileStream(flatFileSinkData.FileName,
                                                         FileMode.Append,
FileAccess.Write, FileShare.ReadWrite))
           {
               using (StreamWriter writer = new StreamWriter(fileStream))
               {
                   lock (syncObject)
                   {
                       if (flatFileSinkData.Header.Length > 0)
                       {
                           writer.WriteLine(flatFileSinkData.Header);
                       }
                       writer.WriteLine(FormatEntry(logEntry));
                       if (flatFileSinkData.Footer.Length > 0)
                       {
                           writer.WriteLine(flatFileSinkData.Footer);
                       }

                       writer.Flush();
                   }
               }
           }
       }

You see if the filename is short, directory variable is set from current
appDomain base directory whish is perfect but it is not passed to FileStream.
I think the real problem is the FileStream class which stupidly opens the
file to the location openfile dialog last pointed.
Anyways, this is the story.