PHP Error using ssh backup

Hello,
On several FreePBX Servers using ssh backup server, it produces the following PHP error. However the backups are saved on the ssh server. Tried this with 4 separate FreePBX Servers all running same version 10.13.66-16

Initializing Backup 4
Backup Lock acquired!
Running pre-backup hooks…
Adding items…
Building manifest…
Creating backup…
Storing backup…
PHP Fatal error: [] operator not supported for strings in /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 363
Whoops\Exception\ErrorException: [] operator not supported for strings in file /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 363
Stack trace:

  1. () /var/www/html/admin/modules/backup/functions.inc/class.backup.php:363

Thanks.