The ch.qos.logback.access.spi.AccessEvent does not implement ch.qos.logback.classic.spi.ILoggingEvent, so if you try to log access log to syslog you get a ClassCastException during the append(E eventObject) of class SyslogAppenderBase which SyslogAppender inherits from. The actual ClassCastException happens when calling Converter.write(buf, event) in the writeLoopOnConverters(E event) method of class PatternLayoutBase<E> which uses the convert(ILoggingEvent event) method of the SyslogStartConverter which you can see expects an ILoggingEvent.