NamingException raised. Message: [unable to find primary

We are using WebLogic 9.2 MP2.

Our configuration consists of a cluster with two managed servers. 

We are encountering NamingException errors looking up EJBs in the cluster from
our web tier. The EJB is contained in an EAR that is targetted at the cluster.
We are also seeing the same problem with an EJB targetted at one of the managed
servers.

A restart of the server fixes the problem. An update of the EAR (through the
console) also appears to resolve it.

Any ideas?

Failed to find object bound to JNDI at [XEntryService]. NamingException raised.
Message: [unable to find primary representative]
        at
com.X.core.common.aquila.ejb.AbstractJNDILookup.getJNDIObject(AbstractJNDILookup
.java:103)
        at
com.X.core.common.aquila.ejb.AbstractJNDILookup.getNarrowedJNDIObject(AbstractJN
DILookup.java:78)
        at
com.X.core.common.aquila.ejb.ServiceLocator.initialise(ServiceLocator.java:111)
        at
com.X.core.common.aquila.ejb.ServiceLocator.getRemoteHome(ServiceLocator.java:85
)
        at
com.X.core.common.impl.web.EJBServiceServerWebProxy$ServiceCacheEntry.getHome(EJ
BServiceServerWebProxy.java:880)
        at
com.X.core.common.impl.web.EJBServiceServerWebProxy$ServiceCacheEntry.access$200
(EJBServiceServerWebProxy.java:846)
        at
com.X.core.common.impl.web.EJBServiceServerWebProxy.getServiceFromHome(EJBServic
eServerWebProxy.java:698)
        at
com.X.core.common.impl.web.EJBServiceServerWebProxy.doPost(EJBServiceServerWebPr
oxy.java:377)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
        at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilt
erChain.java:269)
        at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.
java:188)
        at
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:2
10)
        at
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:1
74)
        at
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
        at
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
        at
org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108
)
        at
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)
        at
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870)
        at
org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConne
ction(Http11BaseProtocol.java:665)
        at
org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:52
8)
        at
org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorker
Thread.java:81)
        at
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:68
5)
        at java.lang.Thread.run(Unknown Source)
Caused by: javax.naming.NamingException: unable to find primary representative
[Root exception is javax.naming.NamingException: unable to find primary
representative]
        at weblogic.rjvm.ResponseImpl.unmarshalReturn(ResponseImpl.java:211)
        at
weblogic.rmi.cluster.ClusterableRemoteRef.invoke(ClusterableRemoteRef.java:338)
        at
weblogic.rmi.cluster.ClusterableRemoteRef.invoke(ClusterableRemoteRef.java:252)
        at weblogic.jndi.internal.ServerNamingNode_922_WLStub.lookup(Unknown
Source)
        at weblogic.jndi.internal.WLContextImpl.lookup(WLContextImpl.java:374)
        at weblogic.jndi.internal.WLContextImpl.lookup(WLContextImpl.java:362)
        at javax.naming.InitialContext.lookup(Unknown Source)
        at
com.X.core.common.aquila.ejb.AbstractJNDILookup.getJNDIObject(AbstractJNDILookup
.java:94)
        ... 23 more
Caused by: javax.naming.NamingException: unable to find primary representative
        at
weblogic.rmi.cluster.ClusterableRemoteObject.getReferent(ClusterableRemoteObject
.java:131)
        at
weblogic.jndi.internal.WLNamingManager.getObjectInstance(WLNamingManager.java:95
)
        at
weblogic.jndi.internal.ServerNamingNode.resolveObject(ServerNamingNode.java:348)

        at
weblogic.jndi.internal.BasicNamingNode.resolveObject(BasicNamingNode.java:856)
        at
weblogic.jndi.internal.BasicNamingNode.lookup(BasicNamingNode.java:209)
        at weblogic.jndi.internal.RootNamingNode_WLSkel.invoke(Unknown Source)
        at weblogic.rmi.internal.BasicServerRef.invoke(BasicServerRef.java:550)
        at
weblogic.rmi.cluster.ClusterableServerRef.invoke(ClusterableServerRef.java:224)
        at weblogic.rmi.internal.BasicServerRef$1.run(BasicServerRef.java:440)
        at
weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.ja
va:363)
        at
weblogic.security.service.SecurityManager.runAs(SecurityManager.java:147)
        at
weblogic.rmi.internal.BasicServerRef.handleRequest(BasicServerRef.java:436)
        at
weblogic.rmi.internal.BasicServerRef.access$300(BasicServerRef.java:58)
        at
weblogic.rmi.internal.BasicServerRef$BasicExecuteRequest.run(BasicServerRef.java
:975)
        at weblogic.work.ExecuteThread.execute(ExecuteThread.java:209)