Why white screen instead of Wordpress admin?

Question

CMS

Admin stopped working. When you go to wp-admin just a white screen. When navigating to wp-login.php "ERROR: Cookies blocked due to unexpected screen output. See the documentation or contact the support forums for help." Renamed plugins folder, didn't help. Renamed the theme, did not help. Removed htaccess, did not help. I tried to turn on the output of errors, they are not displayed. Who met with this problem? Followed the standard instructions.
Here is the log
[22-Dec-2015 02:16:06 UTC] PHP Notice: The called WP_Widget class constructor method is deprecated as of version 4.3.0! Use __construct(). in /var/www/kolosov/data/www/advokat-kras.ru/wp-includes/functions.php on line 3457
[22-Dec-2015 02:16:06 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /var/www/kolosov/data/www/advokat-kras.ru/wp-includes/ post.php:1) in /var/www/kolosov/data/www/advokat-kras.ru/wp-includes/pluggable.php on line 1207

Answer 1

In wp-config.php change:
To:

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

Once again we start the white screen of the admin panel and after that we go to wp-content/debug.log and read the log.

Answer 2

Rule number 1: something does not work > look at the logs.