SSH agent isn't launched after the latest update, complaining about missing records in the known hosts file.
Nevertheless, the records do exist. I've tried to connect manually with ssh, everything was fine, ssh reports that it has found the host in known_hosts.
Probable reason is the hashed host name in known_hosts.
Here is the log from the Jenkins slave launch page:
And this is the debug output from ssh, showing that it has found a record in the second line of known_hosts.
Additional logs from Jenkins system log:
File /var/lib/jenkins/.ssh/known_hosts contains strings, looking like being base64-encoded, delimited by '|'. Here is the sample.