In simple English, WordPress confirms pingbacks with a blank user-agent.
The latest version of Bad Behavior blocks sites with send a blank user agent, as this is a common technique being used by spammers to get through user agent filters.
I faced a similar problem with Silktide and added their IP to the whitelist. They plan on fixing this in a future release. I don’t know when that will be and if they will actually do it.
I’m glad WordPress developers have taken this seriously and fixed it. Though there isn’t a major release for this because this is just a minor bug fix, WordPress users please do go and apply the fix.
It isn’t very difficult. All you have to do is open functions.php.
$head = "HEAD $file HTTP/1.1rnHOST: $hostrnrn";
and replace it with
$head = "HEAD $file HTTP/1.1rnHOST: $hostrnUser-Agent: WordPress/" . $wp_version . " PHP/" . phpversion() . "rn";