When Jenkins starts remote slaves via SSH, it does not detect an installed java version and proceeds to download and install JDK7, which it no longer works with. This prevents the slaves from starting and Jenkins from working.
I have configured a Java 1.8 JDK in the global tool config, but that seems to have made no difference.
After removing both jdk.sh and jdk from $JENKINS_HOME on the slaves, the slave still installs JDK7.
I have rolled back to v2.53, which still works fine.
For reference, one of the slaves has JDK1.8.0 installed via the OS and that slave connects and runs ok.
Edit: I changed the component to ssh-slaves-plugin, as that seems to be the culprit.