Where is the mesg? In the online.log or syslog? I have never seen this in online.log. I didn't think it captured any OS specific data.
What version of informix?
If it occurs during an ontape -c process, are you killing the ontape, logging out, etc? It sounds to me like the ontape is working but possibly losing it's connection. As I understand it the standard ontape command requires a dedication session, you can't put it in the background, nohup it, cron, etc.
If you post more details...
Also, for folks infinitely more knowledgeble about informix, try posting to the informix user group or searching the informix user group archives.
The list is very very active, just be sure to post all the details of what you are doing, and don't post mime or you'll be flamed to the center.
hth
Ddoug