When upgrading from ZK 6.0.0 to 6.0.1, I started getting the following exception:

Jun 23, 2012 11:29:45 AM org.apache.catalina.core.AprLifecycleListener init
INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: C:\Program Files\Java\jdk1.7.0_05\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;C:\Program Files\Java\jdk1.7.0_05\jre\bin;C:/Progra~1/Java/jdk1.7.0_03//bin/../jre/bin/server;C:/Progra~1/Java/jdk1.7.0_03//bin/../jre/bin;C:/Progra~1/Java/jdk1.7.0_03//bin/../jre/lib/amd64;C:\Program Files (x86)\Informatica\UMQ_5.3\Win2k-i386\bin;C:\Program Files\Informatica\UMQ_5.3\Win2k-x86_64\bin;C:\Program Files (x86)\PC Connectivity Solution\;C:\Program Files\Common Files\Microsoft Shared\Windows Live;C:\Program Files (x86)\Common Files\Microsoft Shared\Windows Live;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\Windows Live\Shared;C:\Progra~1\Java\jdk1.7.0_03\\bin;D:\MinGW\bin;D:\MinGW\msys\1.0\bin;C:\Progra~1\PostgreSQL\9.1\bin;D:\work\pontus\tech\thirdparty\apache-maven-3.0.4\bin;C:\Program Files\TortoiseSVN\bin;C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin;C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE;C:\Program Files (x86)\Calibre2\;D:\work\pontus\tech\thirdparty\apache-ant-1.8.3\bin;D:\work\thirdparty\eclipse;;.
Jun 23, 2012 11:29:45 AM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]

{Server/Service/Engine/Host/Context}

Setting property 'source' to 'org.eclipse.jst.jee.server:vision' did not find a matching property.
Jun 23, 2012 11:29:45 AM org.apache.coyote.AbstractProtocolHandler init
INFO: Initializing ProtocolHandler ["http-bio-8080"]
Jun 23, 2012 11:29:45 AM org.apache.coyote.AbstractProtocolHandler init
INFO: Initializing ProtocolHandler ["ajp-bio-8009"]
Jun 23, 2012 11:29:45 AM org.apache.catalina.startup.Catalina load
INFO: Initialization processed in 799 ms
Jun 23, 2012 11:29:45 AM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina
Jun 23, 2012 11:29:45 AM org.apache.catalina.core.StandardEngine startInternal
INFO: Starting Servlet Engine: Apache Tomcat/7.0.12
Jun 23, 2012 11:29:49 AM org.zkoss.zk.ui.http.WebManager <init>:115
INFO: Starting ZK 6.0.1 CE (build: 2012050214)
Jun 23, 2012 11:29:49 AM org.zkoss.zk.ui.sys.ConfigParser parseConfigXml:160
INFO: Loading system default
Jun 23, 2012 11:29:49 AM org.zkoss.util.resource.ClassLocator getDependentXMLResources:80
WARNING: Replicate resource: zk
Overwrite jar:file:/C:/Users/leo/.m2/repository/org/zkoss/zk/zk/6.0.1/zk-6.0.1.jar!/metainfo/zk/config.xml
with jar:file:/D:/work/pontus/tech/inhouse/gui/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/vision/WEB-INF/lib/zk.jar!/metainfo/zk/config.xml
Jun 23, 2012 11:29:49 AM org.zkoss.util.resource.ClassLocator getDependentXMLResources:80
WARNING: Replicate resource: zkbind
Overwrite jar:file:/C:/Users/leo/.m2/repository/org/zkoss/zk/zkbind/6.0.1/zkbind-6.0.1.jar!/metainfo/zk/config.xml
with jar:file:/D:/work/pontus/tech/inhouse/gui/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/vision/WEB-INF/lib/zkbind.jar!/metainfo/zk/config.xml
Jun 23, 2012 11:29:49 AM org.zkoss.zk.ui.sys.ConfigParser checkVersion:126
INFO: Ignore jar:file:/D:/work/pontus/tech/inhouse/gui/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/vision/WEB-INF/lib/zk.jar!/metainfo/zk/config.xml
Cause: version not matched; expected=6.0.1, xml=6.0.0
Jun 23, 2012 11:29:49 AM org.zkoss.zk.ui.sys.ConfigParser checkVersion:126
INFO: Ignore jar:file:/D:/work/pontus/tech/inhouse/gui/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/vision/WEB-INF/lib/zkbind.jar!/metainfo/zk/config.xml
Cause: version not matched; expected=6.0.1, xml=6.0.0
Jun 23, 2012 11:29:49 AM org.zkoss.zk.ui.sys.ConfigParser parse:267
INFO: Parsing jndi:/localhost/vision/WEB-INF/zk.xml
Jun 23, 2012 11:29:52 AM org.zkoss.util.resource.impl.LabelLoader loadLabels:232
INFO: Loading labels for en_US
Jun 23, 2012 11:29:52 AM org.zkoss.util.resource.impl.LabelLoader loadLabels:232
INFO: Loading labels for en
Jun 23, 2012 11:29:52 AM org.zkoss.zk.ui.metainfo.DefinitionLoaders load0:179
SEVERE: Failed to load jar:file:/C:/Users/leo/.m2/repository/org/zkoss/zk/zul/6.0.1/zul-6.0.1.jar!/metainfo/zk/lang.xml
org.zkoss.zk.device.DeviceNotFoundException: ajax not found.
at org.zkoss.zk.ui.metainfo.LanguageDefinition.<init>(LanguageDefinition.java:344)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.parseLang(DefinitionLoaders.java:275)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.load0(DefinitionLoaders.java:177)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.load(DefinitionLoaders.java:153)
at org.zkoss.zk.ui.metainfo.LanguageDefinition.init(LanguageDefinition.java:315)
at org.zkoss.zk.ui.metainfo.LanguageDefinition.getAll(LanguageDefinition.java:294)
at org.zkoss.zk.ui.http.WebManager.getCWRURLPrefix(WebManager.java:268)
at org.zkoss.zk.ui.http.WebManager.<init>(WebManager.java:225)
at org.zkoss.zk.ui.http.HttpSessionListener23.contextInitialized(HttpSessionListener23.java:140)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4701)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5204)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5199)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

Jun 23, 2012 11:29:52 AM org.apache.catalina.core.StandardContext listenerStart
SEVERE: Exception sending context initialized event to listener instance of class org.zkoss.zk.ui.http.HttpSessionListener
org.zkoss.zk.device.DeviceNotFoundException: ajax not found.
at org.zkoss.zk.ui.metainfo.LanguageDefinition.<init>(LanguageDefinition.java:344)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.parseLang(DefinitionLoaders.java:275)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.load0(DefinitionLoaders.java:177)
at org.zkoss.zk.ui.metainfo.DefinitionLoaders.load(DefinitionLoaders.java:153)
at org.zkoss.zk.ui.metainfo.LanguageDefinition.init(LanguageDefinition.java:315)
at org.zkoss.zk.ui.metainfo.LanguageDefinition.getAll(LanguageDefinition.java:294)
at org.zkoss.zk.ui.http.WebManager.getCWRURLPrefix(WebManager.java:268)
at org.zkoss.zk.ui.http.WebManager.<init>(WebManager.java:225)
at org.zkoss.zk.ui.http.HttpSessionListener23.contextInitialized(HttpSessionListener23.java:140)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4701)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5204)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5199)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

Jun 23, 2012 11:29:52 AM org.apache.catalina.core.StandardContext startInternal
SEVERE: Error listenerStart
Jun 23, 2012 11:29:52 AM org.apache.catalina.core.StandardContext startInternal
SEVERE: Context [/vision] startup failed due to previous errors
Jun 23, 2012 11:29:52 AM org.apache.coyote.AbstractProtocolHandler start
INFO: Starting ProtocolHandler ["http-bio-8080"]
Jun 23, 2012 11:29:52 AM org.apache.coyote.AbstractProtocolHandler start
INFO: Starting ProtocolHandler ["ajp-bio-8009"]
Jun 23, 2012 11:29:52 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 7453 ms

I narrowed it down to the ajax class definition in the zk.xml file not having a default class type:

In the 6.0.0 version, the following worked without setting a device-class:

<zk>
<device-config>
<device-type>ajax</device-type>
<server-push-class>org.zkoss.zk.ui.impl.PollingServerPush
</server-push-class>
<timeout-uri>/timeout.zul</timeout-uri><!-- An empty URL can cause the
browser to reload the same URL -->
</device-config>
....
</zk>

In 6.0.1, I had to add teh device-class explicitly (even though the doc seems to mention that this is an optional field:

<zk>
<device-config>
<device-type>ajax</device-type>
<device-class>org.zkoss.zk.device.AjaxDevice</device-class>
<server-push-class>org.zkoss.zk.ui.impl.PollingServerPush
</server-push-class>
<timeout-uri>/timeout.zul</timeout-uri><!-- An empty URL can cause the
browser to reload the same URL -->
</device-config>
....
</zk>