It relates because cron send output to /dev/null so no mail is sent, etc. With /dev/null giving Permission denied whenever trying to run cron, cron was failing as well - freshclam was not updating clamAV.
Now that /dev/null is now fixed, all completes, no errors. In search for why this happen, it is due to an incorrect handling of /dev/null where it actually writes a file.
Since this happened 7 or 8 days ago, across my entire cluster, as well as on servers with NO clients, i tend to think it was kloxoMR related. It is only theory.