Thanks for the WP_DEBUG nfo. It may explain certain 405 errors.
wp-config.php file: define('WP_DEBUG', false);
The latest nfo (<time datetime=”2016-06-22T11:58:58+00:00″> Jun 22, 2016, 1:58 PM) </time>concerning @pjbals.co.pl
Source: https://en.forums.wordpress.com/topic/suspicious-follows?replies=35
Emails from that domain have already been blocked from mass-following any more WordPress.com sites.
Thanks everyone who reported this.
Some of you might still get notifications from pending follows that were started before we were able to block this, should those people click the confirmation link in the subscription emails. We don’t have a way to prevent that, but new follows should not be possible anymore for these users.