Cannot CTRL+click any symbol in a Spock specification (NoClassDefFoundError)

I cannot CTRL+click any symbol in a Spock specification. Sometimes CTRL+clicking does nothing (symbol doesn't turn into a link when I hover over it), sometimes it opens a pop-up "Go To Declaration | Go To Implementation", but that doesn't really work either.
As soon as I hover over a symbol with CTRL held, the error log is flooded with the following entry:

eclipse.buildId=M20090917-0800
java.version=1.6.0_17
java.vendor=Apple Inc.
BootLoader constants: OS=macosx, ARCH=x86_64, WS=cocoa, NL=en_US
Framework arguments: -product org.eclipse.epp.package.jee.product -keyring /Users/xxx/.eclipse_keyring -showlocation
Command-line arguments: -os macosx -ws cocoa -arch x86_64 -product org.eclipse.epp.package.jee.product -keyring /Users/xxx/.eclipse_keyring -showlocation

This is a continuation of log file /swd/prj/workspaces/spock-groovy-1.7/.metadata/.bak_1.log
Created Time: 2010-01-09 21:34:50.329

Error
Sat Jan 09 21:39:59 CET 2010
Unhandled event loop exception

java.lang.NoClassDefFoundError: Could not initialize class org.spockframework.compiler.SpockTransform
at sun.reflect.GeneratedConstructorAccessor16.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at java.lang.Class.newInstance0(Class.java:355)
at java.lang.Class.newInstance(Class.java:308)
at org.codehaus.groovy.transform.ASTTransformationVisitor.addPhaseOperationsForGlobalTransforms(ASTTransformationVisitor.java:348)
at org.codehaus.groovy.transform.ASTTransformationVisitor.doAddGlobalTransforms(ASTTransformationVisitor.java:315)
at org.codehaus.groovy.transform.ASTTransformationVisitor.addGlobalTransforms(ASTTransformationVisitor.java:208)
at org.codehaus.groovy.transform.ASTTransformationVisitor.addPhaseOperations(ASTTransformationVisitor.java:171)
at org.codehaus.groovy.control.CompilationUnit.<init>(CompilationUnit.java:207)
at org.codehaus.jdt.groovy.internal.compiler.ast.GroovyParser.<init>(GroovyParser.java:142)
at org.codehaus.jdt.groovy.internal.compiler.ast.GroovyParser.<init>(GroovyParser.java:94)
at org.codehaus.jdt.groovy.integration.internal.MultiplexingSourceElementRequestorParser.<init>(MultiplexingSourceElementRequestorParser.java:67)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.buildStructure(GroovyCompilationUnit.java:295)
at org.eclipse.jdt.internal.core.Openable.generateInfos(Openable.java:258)
at org.eclipse.jdt.internal.core.JavaElement.openWhenClosed(JavaElement.java:515)
at org.eclipse.jdt.internal.core.CompilationUnit.makeConsistent(CompilationUnit.java:1094)
at org.eclipse.jdt.internal.core.ReconcileWorkingCopyOperation.makeConsistent(ReconcileWorkingCopyOperation.java:170)
at org.eclipse.jdt.internal.core.ReconcileWorkingCopyOperation.executeOperation(ReconcileWorkingCopyOperation.java:89)
at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:728)
at org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(JavaModelOperation.java:788)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.reconcile(GroovyCompilationUnit.java:430)
at org.eclipse.jdt.internal.core.CompilationUnit.reconcile(CompilationUnit.java:1213)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.getModuleNode(GroovyCompilationUnit.java:176)
at org.codehaus.groovy.eclipse.codebrowsing.requestor.CodeSelectHelper.select(CodeSelectHelper.java:36)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.codeSelect(GroovyCompilationUnit.java:525)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.codeSelect(GroovyCompilationUnit.java:517)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.codeSelect(GroovyCompilationUnit.java:512)
at org.eclipse.jdt.internal.ui.javaeditor.JavaElementHyperlinkDetector.detectHyperlinks(JavaElementHyperlinkDetector.java:67)
at org.eclipse.ui.texteditor.HyperlinkDetectorRegistry$HyperlinkDetectorDelegate.detectHyperlinks(HyperlinkDetectorRegistry.java:80)
at org.eclipse.jface.text.hyperlink.HyperlinkManager.findHyperlinks(HyperlinkManager.java:263)
at org.eclipse.jface.text.hyperlink.HyperlinkManager.mouseMove(HyperlinkManager.java:439)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:199)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:3543)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1250)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1273)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1079)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3441)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3100)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
at org.eclipse.equinox.launcher.Main.run(Main.java:1311)