UPDATED on Mar 26, 2016: We added 5 new servers and moved all blocked cronjobs there. They're all working fine now. Our 3 old IP addresses are also unblocked.
SetCronJob's IP addresses have been being blocked by HostGator's firewall for days. If your sites are hosted at HostGator, you can see this error message
Access denied. Your IP address is blacklisted. If you feel this is in error please contact your hosting provider's abuse department.
in cronjobs' execution results.
So, currently we'll disable all cronjobs for sites hosted at HostGator. I'll update when they unblock our IP addresses.
To prevent this from happening again, please lower your cronjob's execution interval, especially when your websites are hosted at shared hosting providers. Most cronjobs don't require cronjob every minute to work. You can set it to run every 10 minutes instead, or contact me for advice.
You should also contact HostGator and ask them if they can whitelist our IP addresses: