Turns out I was the culprit after all. I searched through all my files for domain1.com and sure enough on lines 7/8 of wp-config.php, I found these lines (which I completely forgot about):
define( 'WP_CONTENT_URL', 'http://domain1.com/customlogin' );
define( 'WP_CONTENT_DIR', '/user/domain1.com/customlogin' );