@tom-j-nowell Thanks for your reply… Late last, night I finally cracked the problem using this solution, which I advise everyone to add to their dev notes if you have to migrate wp sites:
solution:
define('WP_CONTENT_DIR', ABSPATH . 'wp-content/' );
define( 'WP_PLUGIN_DIR', WP_CONTENT_DIR . '/plugins' );
In my case, I migrated a multisite from a LAMP environment that previously had the usual root path /var/www/html/wordpress-directory/wp-content to an ISPConfig jailkit environment /web/wordpress-directory/wp-content.