OPSCenter reports sent by email don't attach to message

Hi, I'm having some problems with the generation and sending reports by email in PDF format. I've got 3 schedules to generate and send 3 different reports, but since about 5 days only 1 of those emails has the report attached (randomly, some days only report1 has the attach report, some days report2 or report3). And the emails which haven't the report attached shows the follow message ah the botom of the email:

Unknown error while running the scheduled report <report_name> due to null

Checking OPSCenter logs, I've found the follow messages:

Error while running the scheduled report <report_name> Exception: java.util.ConcurrentModificationException at java.util.ArrayList.sort(ArrayList.java:1456) at java.util.Collections.sort(Collections.java:175) at com.symantec.nbu.nom.report.reader.BaseReportDefinitionReader.getApplicableTableJoinSet(BaseReportDefinitionReader.java:129) at com.symantec.nbu.nom.report.reader.custom.CustomReportDefinitionReader.getCustomRankingView(CustomReportDefinitionReader.java:645) at com.symantec.nbu.nom.report.reader.custom.CustomReportDefinitionReader.fetchReportDefinition(CustomReportDefinitionReader.java:410) at com.symantec.nbu.nom.report.reader.custom.CustomReportDefinitionReader.fetchReportDefinition(CustomReportDefinitionReader.java:289) at com.symantec.nbu.nom.report.reader.custom.CustomReportDefinitionReader.fetchReportDefinition(CustomReportDefinitionReader.java:95) at com.symantec.nbu.nom.report.reader.ReportDefinitionProvider.fetchCustomReportDefinition(ReportDefinitionProvider.java:216) at com.symantec.nbu.nom.report.reader.ReportDefinitionProvider.fetchReportDefinition(ReportDefinitionProvider.java:115) at com.symantec.nbu.nom.report.ReportDefinitionManager.fetchReportDefinition(ReportDefinitionManager.java:35) at com.symantec.nbu.nom.report.common.ReportExportHelper.isExportFormatSupported(ReportExportHelper.java:187) at com.symantec.nbu.nom.report.executor.ReportTask.run(ReportTask.java:97) at com.netbackup.common.threadpool.ThreadDecorator.run(ThreadDecorator.java:52) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745) ,40:com.symantec.nbu.nom.util.JavaLogger.app,0,(0|)

Could anyone help me with this issue? Thanks in advance.

Claudio.