Much of the time when a crontab fails to run it's due to a bad $PATH variable. That is, cron has a shorter $PATH than everything else, so I always, if running a script from cron put complete paths in, such as /usr/bin/whatever. (I put this into the script that it's running.)
That's a wild guess though, since this sounds like something that used to work but no longer works. Would you be a bit more specific? For example, are the failed jobs for the same user, a different user? Could it be that the user lacks privilege?
I think I'd try to test it with something really simple, like have it run ls and write it to something to make sure that it at least works. (e.g. ls /var > var.txt)
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