Upstream bug JENKINS-25412.
Jenkins master could not connect on slaves (T100509). Moritz explanation:
The hardened sshd MAC and KEX setup not being supported by the jsch package which is embedded in the jenkins package. According to the website http://www.jcraft.com/jsch/ the current release supports aes256-ctr as the cipher and diffie-hellman-group-exchange-sha256 as the kex, which would make it
Hence this bug.