IllegalMonitorStateException when applet is shut down

Issue

The following exception occurs when our applet is shut down

Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(Unknown Source)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(Unknown Source)
    at java.util.concurrent.locks.ReentrantLock.unlock(Unknown Source)
    at java.util.concurrent.DelayQueue.take(Unknown Source)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(Unknown Source)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.getTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)