Hello Jürgen,
what commands did you use? I don't know the specifics of the different cron
daemons in *nix, however if you executed "crontab -e" you will need the
EDITOR environment variable defined *inside* the chroot and the editor (vim,
nano, emacs, joe or whatever) must reside inside the jail.
I suppose you want to run a separate cron daemon process inside the jail?
"man cron" (if you use vixie cron) seems to be a reliable source of what
files the cron daemon reads and would probably help you set up the cron
daemon within the jail.
Special consideration must be awarded to the fact that you somehow will have
to start the cron daemon within the jail after starting the computer, so you
will probably have to copy and hack away at the cron daemon init script that
comes with your OS / Linux distribution to get the cron daemon started
inside the jail.
Also, I have no idea if the cron daemon needs access to special /device
files (such as a timer), so you would have to connect that device file from
outside the jail to the inside of the jail. And I haven't the slightest clue
if cron daemons are so intertwined with the kernel, that having them inside
the jail might be too much of a security risk (if you run a webserver or
similar inside the jail).