Make sure you:
- run
screen -ls
as the same user that is used by crontab to start your command - use a main command that doesn't end before you reattach, since that will end your session as well. If you run a short command and want the session to persist, use something like
bash -c 'short_command; exec bash'
(or, as pointed out in the link you mention, adddefzombie ZZ
to your.screenrc
).