Checked the server.log files and found the following error:
2006-08-16 13:07:18,906 ERROR [com.atlassian.jira.event.issue.IssueEventDispatcher] Exception thrown from listener [com.atlassian.jira.event.listeners.ParentIdSubTaskListener] : No event type with id 777
java.lang.IllegalArgumentException: No event type with id 777
at com.atlassian.jira.event.type.DefaultEventTypeManager.getEventType(DefaultEventTypeManager.java:100)
at com.atlassian.jira.event.issue.AbstractIssueEventListener.workflowEvent(AbstractIssueEventListener.java:178)
at com.atlassian.jira.event.issue.IssueEventDispatcherSkidata.dispatchEvent(IssueEventDispatcherSkidata.java:84)
at com.atlassian.jira.event.issue.IssueEventDispatcherSkidata.dispatchEvent(IssueEventDispatcherSkidata.java:58)
at com.atlassian.jira.event.issue.IssueEventDispatcherSkidata.dispatchEvent(IssueEventDispatcherSkidata.java:41)
at com.atlassian.jira.web.action.issue.LinkExistingIssue.doExecute(LinkExistingIssue.java:142)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:58)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:132)
at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:178)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.AccessLogFilter.doFilter(AccessLogFilter.java:35)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:119)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:55)
at com.atlassian.jira.web.filters.SitemeshExcludePathFilter.doFilter(SitemeshExcludePathFilter.java:38)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:177)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132)
at com.atlassian.jira.web.filters.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:25)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.gzip.GzipFilter.doFilter(GzipFilter.java:72)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:175)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:74)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.authenticator.SingleSignOn.invoke(SingleSignOn.java:392)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.MasterSlaveWorkerThread.run(MasterSlaveWorkerThread.java:112)
at java.lang.Thread.run(Thread.java:595)

What is the cause of this error and how can I fix it?
Thx, Peter