Fastpath is not starting in the latest build. Same was when we tried to update to Smack 3.4.0. Fastpath part was inside Smack before, but since 3.4.0 it was moved out into separate jar smackx-workgroups.
I captured this today while playing around. it looks like it fails in two place. Hope this helps
Aug 05, 2016 8:50:53 PM org.jivesoftware.spark.util.log.Log debug INFO: Trying to initialize org.jivesoftware.fastpath.FastpathPlugin@1896ad9
Aug 05, 2016 8:50:53 PM org.jivesoftware.spark.util.log.Log debug INFO: Took 4 ms. to load org.jivesoftware.fastpath.FastpathPlugin@1896ad9
Aug 05, 2016 8:50:53 PM org.jivesoftware.spark.util.log.Log debug INFO: Trying to initialize org.jivesoftware.sparkimpl.plugin.viewer.PluginViewer[,0,0,0x0,invalid,layout=java.awt.GridBagLayout,alignmentX=0.0,alignmentY=0.0,border=,flags=9,maximumSize=,minimumSize=,preferredSize=]
Aug 05, 2016 8:50:53 PM org.jivesoftware.spark.util.log.Log debug INFO: Took 8 ms. to load org.jivesoftware.sparkimpl.plugin.viewer.PluginViewer,0,0,0x0,invalid,layout=java.awt.GridBagLayout,alignmentX=0.0,alignmentY=0.0,border=,flags=9,maximumSize=,minimumSize=,preferredSize=]
Exception in thread "Thread-22" java.lang.ClassCastException: org.jivesoftware.smack.packet.UnparsedIQ cannot be cast to org.jivesoftware.smackx.workgroup.packet.AgentWorkgroups
at org.jivesoftware.smackx.workgroup.agent.Agent.getWorkgroups(Agent.java:42)
at org.jivesoftware.fastpath.FastpathPlugin$2.construct(FastpathPlugin.java:117)
at org.jivesoftware.spark.util.SwingWorker.lambda$new$1(SwingWorker.java:140)
at java.lang.Thread.run(Thread.java:745)
Merged https://github.com/igniterealtime/Spark/pull/188
Fastpath plugin now loads.