SwiftMail not sending to Mandrill

I updated Yii2 and it’s components yesterday and now I can’t send email to Mandrillapp. It works fine with Mailgun though. This is the error that Mandrill is showing:

Invalid raw_message: Bad multipart message - start missing

Was something changed on Swiftmail since 2.0 was released?

Any ideas on this?

There is another thread on swiftmailer, maybe it is related.