GOTCHA!
problem was in a backup script found on the forums! (
Zimbra Backup Script ... Small Problem )
Should have checked it better before implementing it :-(... It stopped zimbra and restarted it without sourcing all the env stuff the zimbra user has. By using "su - zimbra -c" (-i flag on sudo would work as well) stats service would finally get restarted correctly by cron. Here's a snippet of the stopping section, but the situation was identical in the starting part too.
Code:
[...]
# Stop Zimbra Services
# bad method
# sudo -u zimbra $ZIMBRA_DIR/bin/zmcontrol stop
# working method
su - zimbra -c 'zmcontrol stop'
[...]
Thanks for your support, folks!