java.lang.NoClassDefFoundError: com/springsource/sts/maven/MavenCorePlugin$PomResourceChangeListener$PomResourceVisitor

I upgrade from STS 2.8.0 RELEASE to 2.8.1.RELEASE a few days ago. Everything's been working fine. But today, suddenly, I've got an error with description "null" in my Problems view for the pom.xml of all of my projects. My Error Log is showing these errors:

Message: Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".

java.lang.NoClassDefFoundError: com/springsource/sts/maven/MavenCorePlugin$PomResourceChangeListener$PomResourceVisitor
at com.springsource.sts.maven.MavenCorePlugin$PomResourceChangeListener.getVisitor(MavenCorePlugin.java:203)
at com.springsource.sts.maven.MavenCorePlugin$PomResourceChangeListener.resourceChanged(MavenCorePlugin.java:174)
at org.eclipse.core.internal.events.NotificationManager$1.run(NotificationManager.java:291)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.NotificationManager.notify(NotificationManager.java:285)
at org.eclipse.core.internal.events.NotificationManager.broadcastChanges(NotificationManager.java:149)
at org.eclipse.core.internal.resources.Workspace.broadcastPostChange(Workspace.java:395)
at org.eclipse.core.internal.resources.Workspace.endOperation(Workspace.java:1530)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:156)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:241)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:54)

Session Data:

eclipse.buildId=2.8.1.201111221000-RELEASE
java.version=1.7.0_01
java.vendor=Oracle Corporation
BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=en_US
Framework arguments: -product com.springsource.sts.ide
Command-line arguments: -os win32 -ws win32 -arch x86_64 -product com.springsource.sts.ide