¿Qué causa este tipo de error en Tomcat?

SEVERE: Exception loading sessions from persistent storage
java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException:
   bean.ProjectAreaBean
 at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1333)
 at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
 at java.util.ArrayList.readObject(ArrayList.java:593)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(
    DelegatingMethodAccessorImpl.java:25)