Re: [4.4.2]Error on automated backup, core\task\course_backup_task,array_key_exists(): Argument #2 ($array) must be of type array, null given

That's a PHP setting.  Go to Admin, Server, PHP info and look for max's settings - maximum execution time to be specific.

Right now, that's set to 480 seconds ... increase that ... can't tell you to what value, but considering this is a backup of what could be a large course ... guess: 580 seconds.   If that doesn't do it on same course backup, increase it some more ... 680 seconds.

Change to php settings requires restart of web services.

BTW, I still recommend running of check_schema script in admin/cli/

Your server could have both issues ... time and db schema.

'SoS', Ken