I have got the following exception when starting a job, any idea?

Code:
Kettle Run Job Error: null: java.lang.NullPointerException
	at org.pentaho.di.core.logging.Log4jBufferAppender.getLogBufferFromTo(Log4jBufferAppender.java:87) [kettle-core.jar:4.4.3]
	at org.pentaho.di.core.logging.Log4jBufferAppender.getLogBufferFromTo(Log4jBufferAppender.java:152) [kettle-core.jar:4.4.3]
	at org.pentaho.di.core.logging.Log4jBufferAppender.getBuffer(Log4jBufferAppender.java:158) [kettle-core.jar:4.4.3]
	at org.pentaho.di.core.logging.Log4jBufferAppender.getBuffer(Log4jBufferAppender.java:171) [kettle-core.jar:4.4.3]
	at org.pentaho.di.core.logging.Log4jBufferAppender.getBuffer(Log4jBufferAppender.java:167) [kettle-core.jar:4.4.3]
	at org.pentaho.di.job.Job.execute(Job.java:617) [kettle-engine.jar:4.4.3]
	at org.pentaho.di.job.Job.execute(Job.java:495) [kettle-engine.jar:4.4.3]
	at org.pentaho.di.job.Job$execute.call(Unknown Source)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:45) [groovy-all-2.0.7.jar:2.0.7]
	at org.pentaho.di.job.Job$execute.call(Unknown Source)