logical logs apparently full

Hi everybody, We are experiencing an intermittent problem backing up logical logs and I wondered if anyone could shed any light on it: The following message appears in the Online log while the logical logs are being backed up by onbar -l into a Veritas Netbackup storage manager: 06:00:23 Logical Log Files are Full -- Backup is Needed 06:00:24 Process exited with return code 1: /bin/sh /bin/sh -c /opt/informix/etc/log_full.sh 3 20 "Logical Logs are Full -- Backup is Needed." "Logical Log Files are and of course the instance hangs. The problem can only be resolved by restarting the engine. "onstat -l" shows the logs all backed up. "onstat -u" shows a process with a "M" in the last column of flags. "onstat -g ses" of the onstat -u "M" session Id shows it to be an ontape, (please note that onbar runs as ontape on the engine). There is no parent onbar or ontape process at the unix level. Environment: Informix server 7.24.UC4X3 HP-UX 10.20 Informix and Veritas are investigating, but not getting very far. You might say upgrade the engine, but we are running such an old version of the engine because it has had modifications coded into it specially for us, so upgrading the (250+) instances is not a straightforward matter. Thanks.