When slave executes ant task, the following occurs.

FATAL: Could not initialize class hudson.util.jna.Advapi32
java.lang.NoClassDefFoundError: Could not initialize class hudson.util.jna.Advapi32
at hudson.util.jna.RegistryKey.open(RegistryKey.java:211)
at hudson.util.jna.RegistryKey.openReadonly(RegistryKey.java:206)
at hudson.plugins.cygpath.CygpathLauncherDecorator$1.getCygwinRoot(CygpathLauncherDecorator.java:82)
at hudson.plugins.cygpath.CygpathLauncherDecorator$1.cygpath(CygpathLauncherDecorator.java:97)
at hudson.plugins.cygpath.CygpathLauncherDecorator$1.launch(CygpathLauncherDecorator.java:63)
at hudson.Launcher$ProcStarter.start(Launcher.java:268)
at hudson.Launcher$ProcStarter.join(Launcher.java:275)
at hudson.tasks.Ant.perform(Ant.java:207)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:583)
at hudson.model.Build$RunnerImpl.build(Build.java:165)
at hudson.model.Build$RunnerImpl.doRun(Build.java:132)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:417)
at hudson.model.Run.run(Run.java:1179)
at hudson.matrix.MatrixRun.run(MatrixRun.java:130)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:122)

Uninstalling the cygpath plugin, fixed the problem.