I assume you are using a pattern based logfile and no xml formatted logfile?
If so, can you actually see the username beeing present when you open the file with a simple text editor?
Are you using the same pattern to read the logfile in Log4View as has been used to generate the file? You can find the pattern used to generate the file in your logging applications' NLog settings in the layout property of your target configuration.
If this still does not work, could you please send us a snippet of your logfile and the pattern you use to firstname.lastname@example.org?
when you change target type to File with the same layout and open the file in an editor, you can see that there is simply no username present in the messages. That's why the username can not be displayed in Log4View.
It seems like NLog does not support logging the username when using the log4jxmlevent format. Yet solution aproaches are discussed here.