I created a webapp and it runs with Tomcat 5.5. I use log4j for logging with multiple appenders (console, rolling files and syslog) and the RollingFileAppender doesn't roll the file ; it always rewrite the same file. But today, I noticed something : it only happens when I reload or stop/start the webapp with Tomcat manager.
Tomcat has a known issue with leaving classes in a corrupt state on reload (http://issues.apache.org/bugzilla/show_bug.cgi?id=41059). Could you either apply the patch on that thread or set the org.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES system property to false.