I just spent several days pulling my hair out trying to figure out the Unknown Connection Error issue. I am testing with the latest version of Spark (2.8.3) and the latest version of Openfire (4.3.2).
I tested in both using a docker container and a VM. Initially, I was using the default-jdk and this did not work. When I downgraded to openjdk-8-jre-headless things started working. Anybody else get this experience?