Seeing as nobody responded - here's what we've been able to work out:
In our server environment (at a large Australian university), the
Apache server is restarted regularly - at least once per week, as a
logrotate is executed, as well as whenever any server changes need to
be made. Twapperkeeper's archiving processes do not restart
automatically as Apache restarts.
The solution we've come up with: we've created a copy of the
startarchiving.php script which is identical to startarchiving, except
for removing the code block that checks whether the user is an
administrator (below the '// Ensure user is an administrator' comment
in the PHP script).
This new script is called automatically after Apache restarts (I won't
describe here how to set this up - ask your server admin), and
restarts the archiving processes. This minimises any archiving outages
purely to the time that Apache itself is down.
Hope this helps some people here - certainly was a significant issue
for us at first.
Axel Bruns