The actual configuration for nodes is "Know host file Verification Strategy" (that means ~/.ssh/known_hosts file)
With version 1.17 all slaves work without issue updating to 1.19 no way to connect. I got that error.
I've try also to change the Verification Strategy save and back to Know host (maybe serialisation is changed) but with no luck.
Actual setting for all slaves
A known_hosts file on master
Jesse Glick let me know if you need other file to profile the issue.