Ubuntu Lucid - unattened upgrade

[ubuntu at cgi-net.ch]

Dear list,

I'm administrating various ubuntu clients running ubuntu version 10.04
LTS.
Everything is working fine, except the unattented-upgrade feature.

The was working for some time, but with the latest updates, it stopped
working, because it asked me what I want to-do with the /etc/issue.net
file.
Please note that I have modified this file for legal purpose.

I have no idea why the utility is asking me a questions and paused until I
entered a response, since I'm running the upgrade using the following
command:
aptitude upgrade -y -t lucid-security

BTW: I'm running the job by cron

Does anyone have an idea how to avoid, that behavior?
If you have any questions, or if you require additional information, feel
free to contact me by phone or email.

Thanks and all the best,
Simon