I had difficulties getting cron.php to log progress and, as the issue was on a remote server would have difficulties debugging. As a consequence, I rolled the CiviCRM installation and database back to backups taken before the issue occurred, and then reinstalled the CiviCRM upgrades. The cron job now appears to be working. Fortunately, the problem was on ...
Are the paths in the civicrm.settings.php files each pointing to the right place?
There's one for each of front and backend:
If you just copy and past the paths that are set those two files (e.g. after a server migration) then you might get the error you ...
It's in civicrm_log. Entity_id matches civicrm_contact.id and entity_table='civicrm_contact'. Modified_id is the contact id of who modified it.
Or if you have detailed logging turned on there's also entries in the log_civicrm_xxx tables.