When using undersocre "_" characters in the property "ApplicationName" of the EventLogAppender, it fails to insert the logs into the eventLog without warnings or anything. I searched the internet for any character restrictions and found nothing. Your documentation only says "The string used to distinguish events from different sources." without mentioning any limitations. This could be a bug (which I spent hours to find out) or windows eventLog limitation, either way it should be stated explicitly or resolved.