Bastion doesn't work

Summary:

Bastion refuses connections for unknown reasons

Content (required):

I have a ansible play that sets up a bastion, creates a session and configures ssh for the next play.

This has been working for ages, but suddenly today stopped working.

Everything seems to work fine, until the point where a ssh-connection is attempted, which errors out with:

Failed to connect to the host via ssh: kex_exchange_identification: Connection closed by remote host\r\nConnection closed by UNKNOWN port 65535

On one of my instances I can also ssh in without using a Bastion (but for consistency the ansible plays use Bastion for all instances), and that works fine. So the instance works fine, it is responding to ssh through a different connection, but refuses connection through the Bastion.