I think this is happening:

• Logback is sending a serialized LoggingEvent over the socket
• the LoggingEvent has a reference to the foo.bar.Car instance
• the Eclipse plugin receives the bytes over the wire and tries to
  deserialize the LoggingEvent, this fails because the foo.bar.Car class
  is not on the classpath.

Resolution: I think the SocketAppender should only send the formattedMessage
over the wire and not the argumentArray.
In other words: argumentArray should be declared transient.

Let's see what the LogBack developers think about that ...