Critical error in settings page

Hello, when I’m trynna modify plugin’s settings, I’m receiving notification which says something like:

“A fatal error has occurred on the site. Additional instructions are included in the message sent to the website administrator’s e-mail address.”

Of course, I didn’t receive any email. What should I do?

I had put in wp-config.php this:

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

And of course I cannot see any bug 😛