That's absolutely correct, Doug: the zip is html with this content:
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 363653928 bytes) in /home/admispconfig/ispconfig/web/tools/tools/backup/backup.php on line 157
php.ini has a 128M memory limit (as you can see).
I already run a cron-based backup script for one of the sites, but now with quite a few more sites now it makes more sense (I think) to use the built-in capability, so I've enabled it in config.inc.php, see here: http://howtoforge.org/forums/showthread.php?t=12063
. Now we'll see how it works tonight.