Updating a task assignment error

When i open a task and change the status to Closed the following tomcat error is displayed. If i look at the request view the changes i have made do go through...i also get a similar error when approving approvals.

type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
java.lang.NullPointerException
com.adventnet.servicedesk.tasks.TaskUtils.updateTask(TaskUtils.java:420)
org.apache.jsp.tasks.CUDTask_jsp._jspService(CUDTask_jsp.java:154)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
com.adventnet.servicedesk.filter.EncodingFilter.doFilter(EncodingFilter.java:66)
com.adventnet.authentication.ContextChangeDetector.doFilter(ContextChangeDetector.java:100)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:75)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.