Hey Russell,
Your FTP credentials does not seem to work.

I’d also like to add here that the warning is not coming from our theme.
That also could be coming from a third party plugin also so please try testing for a plugin conflict. You can do this by deactivating all third-party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
You could also try upgrading your PHP version like the user in another thread.
Lastly, try adding this line to your wp-config.php.
@ini_set( 'display_errors', 0 );
That should be below define( 'WP_DEBUG_DISPLAY', false );
Hope that helps.