Seems that large forum posts are causing the forum_cron cronjob to abort, with the above message. Unfortunately, it is also happening during the course backups through the cronjob. So far, files such as:

mode/forum/lib.php
datalib.php
moodlelib.php
backuplib.php

all crash with the "possible integer overlow" error.

The forum posts can go out to as many as 3800 people before crashing. However, it needs to go to 14K people.