Normally the common syslog destination: /var/log/messages. But it doesn't contain any output from the command, only when a job was started. Output from cron jobs normally gets emailed to root.
You could do this to capture the output:
As for the groovy lines, a cron environment is missing some things compared with a login environment. The PATH is much reduced and if your script makes assumptions about the PATH, some commands will not be found.
00 23 * * * /usr/local/groovy-1.5.6/bin/groovy /mnt/ecmsrv/u/redes/scripts/groovy/backupGeral.groovy > /tmp/debug.out 2>&1