@luigivieira Thank for your solution!
I think you can make a pull request to https://github.com/Codeinwp/flat
Could you please help me enable debug on your wp-config.php file and give me results?
Change from:
define( 'WP_DEBUG', false );
To:
define( 'WP_DEBUG', true );
What is your WordPress version? Flat 1.5.7 is working fine with latest version of WordPress.
