The update.sh script isn't really required to be run in order to update DA itself. The update.sh only does some minor housekeeping. THe da-popb4smtp is probably killing itself, just make sure da-popb4smtp is running, past that, don't worry about that error.
If you got the new update.tar.gz file.. extract it, run "./directadmin p", then restart DA. That's the important part of the update. Don't worry as much about the update.sh. All important housekeeping is already executed before line 29 anyway.
John