Update:
So, after contacting SiteGround (SG) Support and several back-and-forth discussions, SG finally confirmed that the issue is being caused by a wp-config.php file entry created by SG.
The wp-config.php file entry they made is:
/** Sets up WordPress vars and included files. **/
@include_once('/var/lib/sec/wp-settings-pre.php'); // Added by SiteGround WordPress management system
@require_once(ABSPATH . 'wp-settings.php');
@include_once('/var/lib/sec/wp-settings.php'); // Added by SiteGround WordPress management system
Temporary Solution:
To fix the issue, simply comment out the following entries (as noted):
/** Sets up WordPress vars and included files. **/
// @include_once('/var/lib/sec/wp-settings-pre.php'); // Added by SiteGround WordPress management system
@require_once(ABSPATH . 'wp-settings.php');
// @include_once('/var/lib/sec/wp-settings.php'); // Added by SiteGround WordPress management system
I am waiting for SG to explain what is the purpose of their file entry and why did they modify our wp-config.php file without out cognizance.
Will advise once I hear back from SG Support.
Cheers!