Whilst Webmin can be handy, I personally prefer to use the commandline for most things. I find it quicker, easier, more powerful and gives better error messages when things go wrong. It also means less chance of issues (Webmin leverages the commandline utilities so essentially adds an extra layer of complexity and an extra layer when things can go wrong).

Regardless, it sounds like Webmin isn't pre-seeding apt to run non-interactively properly. So on face value, it looks like a Webmin bug to me... We don't actually develop Webmin, just package it and pre-install it. So we probably should push the bug report back upstream (to Webmin devs) but I think we should do a little more investigation before we do that. I've opened a bug on our issue tracker so it doesn't get forgotten.

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But that doesn't really help you right now... TBH, I'm not really sure what you should do with your current situation. I vaguely recall that you should be able to tap into an existing terminal session, but even after a quick google I didn't come up with anything that looked like it would do the trick without access to the current session you want to hook into.

So I'd be inclined to back out and retry from the commandline. As that may leave Postfix broken, you may need to run dpkg-reconfigure on the the postfix package. That should give you those same options again, but as you'll already be on the commandline, you'll be able to answer it this time! IIRC our default config uses #2.

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For future reference though, all security updates are auto installed daily on TurnKey Linux servers, via cron-apt. The only time interaction would be required is a reboot to apply kernel updates (if/when they are installed).

You can check if/when updates have been installed by looking at the log file, i.e.:

cat /var/log/cron-apt/log

There should be a few entries for each day, starting with an entry like this (from a local TKL server I have running):

CRON-APT RUN [/etc/cron-apt/config]: Tue Feb  6 20:18:01 AEDT 2018

(Mine auto runs ~8pm local time). Older log files are also in that same directory but are archived, use zcat to read them without decompressing, e.g.:

zcat /var/log/cron-apt/log.1.gz

To see exactly what was done, check the relevant apt logs; /var/log/apt/history.log & /var/log/apt/term.log Again the older (compressed) logs can be read with zcat.

If you wish to manually run the security updates, you can do that like this:

turnkey-install-security-updates

As a general rule, apt-get upgrade should work fine, but personally, unless I have a specific reason to apply non-security updates, I generally avoid it in terms of general maintenance.