Struts redeploy problem: failed while destroying filter

I've deployed a webapp with struts2 on weblogic 8.1/java 1.4.1_02 from a war
file. It works fine, but when I make a redeploy it throws an exception with the
following two stack traces. The NoClassDefFoundError seems strange to me,
because the two jars struts2-core.jar and backpost-util-concurrent.jar (for
retrotranslation of struts2) contain the classes correctly. 

Thanks for any help.

<Failed while destroying filter: struts2.java.lang.NoClassDefFoundError:
org/apache/struts2/util/ObjectFactoryDestroyable
        at
org.apache.struts2.dispatcher.Dispatcher.cleanup(Dispatcher.java:260)
        at
org.apache.struts2.dispatcher.FilterDispatcher.destroy(FilterDispatcher.java:221
)
        at
weblogic.servlet.internal.WebAppServletContext$FilterDestroyAction.run(WebAppSer
vletContext.java:6394)
        at
weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.ja
va:317)
        at
weblogic.security.service.SecurityManager.runAs(SecurityManager.java:118)
        at
weblogic.servlet.internal.WebAppServletContext.destroyFilters(WebAppServletConte
xt.java:5786)
        at
weblogic.servlet.internal.WebAppServletContext.destroy(WebAppServletContext.java
:5682)
        at
weblogic.servlet.internal.ServletContextManager.removeContext(ServletContextMana
ger.java:188)
        at
weblogic.servlet.internal.HttpServer.unloadWebApp(HttpServer.java:740)
        at
weblogic.servlet.internal.WebAppModule.destroyContexts(WebAppModule.java:767)
        at
weblogic.servlet.internal.WebAppModule.rollback(WebAppModule.java:745)
        at
weblogic.j2ee.J2EEApplicationContainer.rollbackModule(J2EEApplicationContainer.j
ava:3057)
        at
weblogic.j2ee.J2EEApplicationContainer.rectifyClassLoaders(J2EEApplicationContai
ner.java:1429)
        at
weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:117
6)
        at
weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:103
1)
        at
weblogic.management.deploy.slave.SlaveDeployer$ComponentActivateTask.prepareCont
ainer(SlaveDeployer.java:2602)
        at
weblogic.management.deploy.slave.SlaveDeployer$ActivateTask.createContainer(Slav
eDeployer.java:2552)
        at
weblogic.management.deploy.slave.SlaveDeployer$ActivateTask.prepare(SlaveDeploye
r.java:2474)
        at
weblogic.management.deploy.slave.SlaveDeployer.processPrepareTask(SlaveDeployer.
java:798)
        at
weblogic.management.deploy.slave.SlaveDeployer.prepareDelta(SlaveDeployer.java:5
07)
        at
weblogic.management.deploy.slave.SlaveDeployer.prepareUpdate(SlaveDeployer.java:
465)
        at
weblogic.drs.internal.SlaveCallbackHandler$1.execute(SlaveCallbackHandler.java:2
5)
        at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:197)
        at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:170)
>

<Failed while destroying filter: struts2.java.lang.NoClassDefFoundError:
edu/emory/mathcs/backport/java/util/Arrays
        at
edu.emory.mathcs.backport.java.util.concurrent.CopyOnWriteArrayList.toArray(Copy
OnWriteArrayList.java:99)
        at
edu.emory.mathcs.backport.java.util.concurrent.CopyOnWriteArrayList.<init>
(CopyOnWriteArrayList.java:35)
        at
com.opensymphony.xwork2.config.ConfigurationManager.setConfigurationProviders(Co
nfigurationManager.java:100)
        at
com.opensymphony.xwork2.config.ConfigurationManager.destroyConfiguration(Configu
rationManager.java:141)
        at
org.apache.struts2.dispatcher.Dispatcher.cleanup(Dispatcher.java:283)
        at
org.apache.struts2.dispatcher.FilterDispatcher.destroy(FilterDispatcher.java:221
)
        at
weblogic.servlet.internal.WebAppServletContext$FilterDestroyAction.run(WebAppSer
vletContext.java:6394)
        at
weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.ja
va:317)
        at
weblogic.security.service.SecurityManager.runAs(SecurityManager.java:118)
        at
weblogic.servlet.internal.WebAppServletContext.destroyFilters(WebAppServletConte
xt.java:5786)
        at
weblogic.servlet.internal.WebAppServletContext.destroy(WebAppServletContext.java
:5682)
        at
weblogic.servlet.internal.ServletContextManager.removeContext(ServletContextMana
ger.java:188)
        at
weblogic.servlet.internal.HttpServer.unloadWebApp(HttpServer.java:740)
        at
weblogic.servlet.internal.WebAppModule.destroyContexts(WebAppModule.java:767)
        at
weblogic.servlet.internal.WebAppModule.rollback(WebAppModule.java:745)
        at
weblogic.j2ee.J2EEApplicationContainer.rollbackModule(J2EEApplicationContainer.j
ava:3057)
        at
weblogic.j2ee.J2EEApplicationContainer.rectifyClassLoaders(J2EEApplicationContai
ner.java:1429)
        at
weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:117
6)
        at
weblogic.j2ee.J2EEApplicationContainer.prepare(J2EEApplicationContainer.java:103
1)
        at
weblogic.management.deploy.slave.SlaveDeployer$ComponentActivateTask.prepareCont
ainer(SlaveDeployer.java:2602)
        at
weblogic.management.deploy.slave.SlaveDeployer$ActivateTask.createContainer(Slav
eDeployer.java:2552)
        at
weblogic.management.deploy.slave.SlaveDeployer$ActivateTask.prepare(SlaveDeploye
r.java:2474)
        at
weblogic.management.deploy.slave.SlaveDeployer.processPrepareTask(SlaveDeployer.
java:798)
        at
weblogic.management.deploy.slave.SlaveDeployer.prepareDelta(SlaveDeployer.java:5
07)
        at
weblogic.management.deploy.slave.SlaveDeployer.prepareUpdate(SlaveDeployer.java:
465)
        at
weblogic.drs.internal.SlaveCallbackHandler$1.execute(SlaveCallbackHandler.java:2
5)
        at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:197)
        at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:170)