To enable debugging in WordPress, you will need to add the following to your site's wp-config.php file:
define( 'WP_DEBUG', true );
if ( WP_DEBUG ) {
@error_reporting( E_ALL );
@ini_set( 'log_errors', true );
@ini_set( 'log_errors_max_len', '0' );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );
define( 'CONCATENATE_SCRIPTS', false );
define( 'SAVEQUERIES', true );
}Your wp-config.php file may already include a line that says define( 'WP_DEBUG', false );. You can remove it, and replace it by the code above.
Once you've done so, try to reproduce the issue you were facing.
Then, check the wp-content/debug.log file for errors in your WordPress installation. You will need to access this file via FTP, much like you have done to edit wp-config.php.
The file should give you some details about the most recent errors on your site, and in what file they occurred.
Once you figure things out, do not forget to replace define( 'WP_DEBUG', true ); by define( 'WP_DEBUG', false ); in wp-config.php to disable debugging.