Extension Resource URL being rewritten
The 'Extension Resource URL' under 'Resource URLs' is the web-accessible URL that points to the extensions directory, not the path to that directory on the server's filesystem. The path on the server belongs in 'CiviCRM Extensions Directory' under 'Directories'.

Edit: for the example shown, the 'Extension Resource URL' is 'https://ffnew/sites/default/extensions' for a 'CiviCRM Extensions Directory' of '/var/www/html/prod/drupal/sites/default/extensions'. This will differ if you're not on Drupal.