Sometimes I get this exception (Firebird 1.5 with the proper Jaybird version), what is the cause of this?

GDS Exception. 335544327. invalid request handle

org.firebirdsql.jdbc.FBSQLException: GDS Exception. 335544327. invalid request handle
at com.sikgraf.dao.OrcamentoDAO.getOcamentoBean(OrcamentoDAO.java:1266)
at com.sikgraf.dao.OrdemServicoDAO.getOrdemServico(OrdemServicoDAO.java:316)
at com.sikgraf.dao.OrdemServicoDAO.getListaOrdensLivres(OrdemServicoDAO.java:883)
at com.sikgraf.ui.NotaFiscalSaidaUI$103.doInBackground(NotaFiscalSaidaUI.java:4285)
at com.sikgraf.ui.NotaFiscalSaidaUI$103.doInBackground(NotaFiscalSaidaUI.java:4279)
at javax.swing.SwingWorker$1.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at javax.swing.SwingWorker.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
at org.firebirdsql.gds.GDSException: invalid request handle
at org.firebirdsql.jgds.GDS_Impl.readStatusVector(GDS_Impl.java:1816)
at org.firebirdsql.jgds.GDS_Impl.receiveResponse(GDS_Impl.java:1769)
at org.firebirdsql.jgds.GDS_Impl.isc_dsql_execute2(GDS_Impl.java:917)
at org.firebirdsql.jca.FBManagedConnection.executeStatement(FBManagedConnection.java:793)
at org.firebirdsql.jdbc.AbstractConnection.executeStatement(AbstractConnection.java:946)
at org.firebirdsql.jdbc.AbstractPreparedStatement.internalExecute(AbstractPreparedStatement.java:499)
at org.firebirdsql.jdbc.AbstractPreparedStatement.executeQuery(AbstractPreparedStatement.java:109)
at com.sikgraf.dao.OrcamentoDAO.getOcamentoBean(OrcamentoDAO.java:953)
at com.sikgraf.dao.OrdemServicoDAO.getOrdemServico(OrdemServicoDAO.java:316)
at com.sikgraf.dao.OrdemServicoDAO.getListaOrdensLivres(OrdemServicoDAO.java:883)
at com.sikgraf.ui.NotaFiscalSaidaUI$103.doInBackground(NotaFiscalSaidaUI.java:4285)
at com.sikgraf.ui.NotaFiscalSaidaUI$103.doInBackground(NotaFiscalSaidaUI.java:4279)
at javax.swing.SwingWorker$1.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at javax.swing.SwingWorker.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)