You seem to suffer from the nr 1 issue with the iTSec plugin.
As a temporary and quick solution disable the Schedule Database Backups setting in the Database Backups section of the iTSec plugin Settings page.
Then you’ll need to analyze\investigate why the backup is running out of memory. You may have some log table(s) in your database with 10000s of records that are pointless to backup and makes the database backup too big for the iTSec plugin Database Backups code to handle.
Deleting the 10000s records or excluding that log table will help complete the backup.
Sometimes increasing the php memory limit will solve the issue.
But in any case it will be a matter of time before the issue returns …
Best and permanent solution would be for iThemes to rewrite\optimize their Database Backups code.
Alternative is to use another plugin for creating backups.
dwinden