Major issue with scheduled tasks

[LewisW]

After many months without issue on May 9th at 1pm UK time all our scheduled tasks stopped working. Nothing was changed on the server on this day, in fact our logs show nobody even logged in to Plesk or SSH'd in to the server.

The user 'root' has a single job, the plesk backup:
3,18,33,48 * * * * /usr/local/psa/admin/sbin/backupmng >/dev/null 2>&1

The user 'mritweb' has several: (taken from /var/spool/cron/mritweb)
30 1 * * * curl http://address-redacted/cron.delsqlcsv.php >/dev/null 2>&1
0 3 * * * curl http://address-redacted/cron.picklist.php >/dev/null 2>&1
0 19 * * * curl http://address-redacted/cron.dbbackup.php >/dev/null 2>&1
0 4 * * mon curl http://address-redacted/cron.invforam.php >/dev/null 2>&1
1 23 * * mon curl http://address-redacted/cron.barcodes.php >/dev/null 2>&1
1 1 * * wed curl http://address-redacted/cron.syl.php >/dev/null 2>&1
5 4 * * * curl http://address-redacted/callback.php
30 3 * * * curl http://address-redacted/cron.dbbackup.php >/dev/null 2>&1
30 2 * * * curl http://address-redacted/cron.averages.php >/dev/null 2>&1

However only the 'root' tasks seem to be enacted by the cron daemon.

Can anyone shed any light on why this may be happening?