On jenkins startup, the Ivy plugin causes the following Error, which terminates the entire jenkins startup process. Seems Ivy somehow refers the hudson/plugins/nant/NantBuilder class. But I have no jenkins Nant plugin, nor the need for it.

10-okt-2011 16:21:30 jenkins.model.Jenkins$6 onAttained
INFO: Loaded all jobs
10-okt-2011 16:22:30 hudson.ExtensionFinder$Sezpoz find
WARNING: Failed to load hudson.ExtensionFinder$GuiceFinder
java.lang.InstantiationException: com.google.inject.internal.util.$ComputationException: com.google.inject.internal.util.$ComputationException: java.lang.NoClassDefFoundError: [Lhudson/plugins/nant/NantBuilder$NantInstallation;
at net.java.sezpoz.IndexItem.instance(IndexItem.java:191)
at hudson.ExtensionFinder$Sezpoz.find(ExtensionFinder.java:399)
etc...
...
Caused by: java.lang.ClassNotFoundException: hudson.plugins.nant.NantBuilder$NantInstallation
at java.net.URLClassLoader$1.run(Unknown Source)
etc...
...
10-okt-2011 16:22:32 jenkins.model.Jenkins$6 onTaskFailed
SEVERE: Failed NullIdDescriptorMonitor.verifyId
java.lang.Error: java.lang.reflect.InvocationTargetException
at hudson.init.InitializerFinder.invoke(InitializerFinder.java:114)
at hudson.init.InitializerFinder$TaskImpl.run(InitializerFinder.java:174)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$5.runTask(Jenkins.java:798)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at hudson.init.InitializerFinder.invoke(InitializerFinder.java:110)
... 8 more
Caused by: java.lang.NullPointerException
at hudson.diagnosis.NullIdDescriptorMonitor.verifyId(NullIdDescriptorMonitor.java:86)
... 13 more
10-okt-2011 16:22:32 hudson.WebAppMain$2 run
SEVERE: Failed to initialize Jenkins

(I left some parts of the long stacktrace, as it contains some jenkins and google lib internals, and the warning repeated again a few times)

The only way to get jenkins start up successfully is by deleting the ivy.hpi file...