DiskStore .throwableSafeExpireElementsIfRequired(620) org.appfuse.model.User Cache: Could not expire elements from disk due to null

I am facing above exception followed by OutOfMemoryException.

JVMDUMP013I Processed Dump Event "systhrow", detail "java/lang/OutOfMemoryError". 

ERROR [Store org.appfuse.model.User Spool Thread] DiskStore.throwableSafeExpireElementsIfRequired(620) | org.appfuse.model.User Cache: Could not expire elements from disk due to null. Continuing...

java.lang.OutOfMemoryError

        at 

java.util.Collections$SynchronizedMap

.

values

(Unknown Source)

        at

net.sf.ehcache.store.DiskStore

.

expireElements

at

net.sf.ehcache.store. DiskStore

.



Exception in thread "QuartzScheduler_QuartzSchedulerThread" java.lang.OutOfMemoryError

        at java.lang.String.<init>(String.java:364)

        at org.quartz.Trigger.getFullName(Trigger.java:435)

        at org.quartz.simpl.TriggerComparator.compare(RAMJobStore.java:1408)

        at java.util.TreeMap.find(Unknown Source)

        at java.util.TreeMap.remove(Unknown Source)

        at java.util.TreeSet.remove(Unknown Source)

        at org.quartz.simpl.RAMJobStore.acquireNextTrigger(RAMJobStore.java:1144)

        at org.quartz.core.QuartzSchedulerThread.run(QuartzSchedulerThread.java:233)

Exception in thread "http-80-16" java.lang.NullPointerException

Looks like some issue in DiskStore cache causing OutOfMemory Exception. Is there any possible way I can twick ehcache.xml parameters?

Below is the DiskStore ehcache setting for User object:

<cache name="org.appfuse.model.User" maxElementsInMemory="2000" eternal="false" timeToIdleSeconds="0" timeToLiveSeconds="7200" overflowToDisk="true"/>

Any suggestions?