I didn’t touch the rules .php files and WordPress plugin.
The algorithm was as follows:
1) Installed the plugin Cerber;
2) Activated;
3) Went into settings and where prescribed page for authorization: wp-admin;
After that the site stopped to load, but you can reach ../wp-login.php that display the authorization page;
4) Solved the problem deleting the plugin via FTP.
5) Deleted manually: cerber_log, cerber_acl, cerber_blocks;
6) Tried again to install the plug-Cerber; the Admin panel is not loaded again, the error was not given.
7) Deleted the plugin via FTP. By the way the error is thrown when deleting a plugin without deactivation in admin. I’m talking about the error: Plugin wp-cerber/wp-cerber.php has been deactivated because of an error: plugin File not found.
8) Made a delete operation in a manual again.
9) Decided to try to delete via the admin panel. Downloaded the plugin without activating it.
10) Removed the plugin.
11) Installed the plugin via the admin panel.
Again the admin area is no longer open.
The reason I do not understand.