Error serialization problem when vaadin work with memcached-session-manager

Currently using only a tomcat server running vaadin application, with the increase in application traffic, we consider the use of multiple tomcat server load balancing, we use memcached-session-manager (https://github.com/magro/memcached -session-manager) to synchronize session data.
vaadin version 7.6.3,  tomcat memcached-session-mananger version 1.9.5.

here is tomcat context.xml config 
    <Manager className="de.javakaffee.web.msm.MemcachedBackupSessionManager"
        memcachedNodes=".....:11211" 
        username="....."  
        password="...."   
        lockingMode="auto"
        sticky="false"
        requestUriIgnorePattern= ".*\.(png|gif|jpg|css|js)$"   
        sessionBackupAsync= "false"      
        sessionBackupTimeout="5000" 
        operationTimeout="10000"
        transcoderFactoryClass="de.javakaffee.web.msm.JavaSerializationTranscoderFactory"   />

when tomcat start, it throw error info :

18-Jul-2016 10:06:13.795 WARNING [http-nio-80-exec-13] de.javakaffee.web.msm.JavaSerializationTranscoder.writeAttributes Cannot serialize session attribute com.vaadin.server.VaadinSession.WshopServlet for session 222ABF3B6BF136DDA94C75E90AE53F95
 java.io.NotSerializableException: com.vaadin.addon.touchkit.settings.TouchKitSettings$1
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at java.util.ArrayList.writeObject(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor77.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteObject(Unknown Source)
    at com.vaadin.event.ListenerMethod.writeObject(ListenerMethod.java:88)
    at sun.reflect.GeneratedMethodAccessor283.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at java.util.HashSet.writeObject(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor258.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at de.javakaffee.web.msm.JavaSerializationTranscoder.writeAttributes(JavaSerializationTranscoder.java:139)
    at de.javakaffee.web.msm.JavaSerializationTranscoder.serializeAttributes(JavaSerializationTranscoder.java:100)
    at de.javakaffee.web.msm.TranscoderService.serializeAttributes(TranscoderService.java:151)
    at de.javakaffee.web.msm.BackupSessionTask.serializeAttributes(BackupSessionTask.java:180)
    at de.javakaffee.web.msm.BackupSessionTask.call(BackupSessionTask.java:110)
    at de.javakaffee.web.msm.BackupSessionTask.call(BackupSessionTask.java:51)
    at de.javakaffee.web.msm.BackupSessionService$SynchronousExecutorService.submit(BackupSessionService.java:347)
    at de.javakaffee.web.msm.BackupSessionService.backupSession(BackupSessionService.java:206)
    at de.javakaffee.web.msm.MemcachedSessionService.backupSession(MemcachedSessionService.java:1086)
    at de.javakaffee.web.msm.RequestTrackingHostValve.backupSession(RequestTrackingHostValve.java:230)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:159)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:124)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:124)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:124)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:124)
    at de.javakaffee.web.msm.RequestTrackingHostValve.invoke(RequestTrackingHostValve.java:124)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

The above error occurs because vaadin session scope object serialization before read from memcached or writing to memcached.

See discussion in http://stackoverflow.com/questions/26161930/java-io-notserializableexception-is-thrown-while-restarting-vaadin-application, which differ from the problems I encountered, if  errors serialization objects is vaadin core package , how to solve ?

Such as: Error in serialization of the application: Class com.vaadin.addon.touchkit.ui.NavigationButton $ 2 must implement serialization.

I'm new to vaadin. but I think vaadin should not work that way. Maybe you should consider stick session solution.