The 2.3 log4j.properties file doesn't specify a file for the FileAppender (because the necessary lines haven't been uncommented). Hence the log files normally generated via the file appender, logs/atlassian-confluence.log, is presently missing?

If you experience this situation and see errors in the logs such as:

log4j:WARN File option not set for appender [confluencelog].
log4j:WARN Are you using FileAppender instead of ConsoleAppender?
log4j:ERROR No output stream or file set for the appender named [confluencelog].

• Please uncomment the following lines in the log4j.properties file located under confluence/WEB-INF/classes and restart Confluence
  (ie remove the #'s)

#log4j.appender.confluencelog.File=${catalina.home}/logs/atlassian-confluence.log
#log4j.appender.confluencelog.MaxFileSize=20480KB
#log4j.appender.confluencelog.MaxBackupIndex=5

Relevant resources

Confluence 2.3 Release Notes