Why do I receive log4j initialization errors in ServiceMix?

Issue

I'm seeing log4j start-up errors in ServiceMix log files:

DEBUG Scheduling task ManagedService Update: pid=org.ops4j.pax.logging
DEBG Running task ManagedService Update: pid=org.ops4j.pax.logging
log4j:WARN No appenders could be found for logger (org.ops4j.pax.logging.pax-logging-service).
log4j:WARN Please initialize the log4j system properly.
log4j:ERROR A "org.apache.log4j.FileAppender" object is not assignable to a "org.apache.log4j.Appender" variable.
log4j:ERROR "org.apache.log4j.FileAppender" was loaded by sun.misc.Launcher$AppClassLoader@32f4a24a.
log4j:ERROR Could not instantiate appender named "out".
log4j:ERROR A "org.apache.log4j.FileAppender" object is not assignable to a "org.apache.log4j.Appender" variable.
log4j:ERROR The class "org.apache.log4j.Appender" was loaded by
log4j:ERROR "org.apache.log4j.FileAppender" was loaded by sun.misc.Launcher$AppClassLoader@32f4a24a.
log4j:ERROR Could not instantiate appender named "out".