Constant Spark disconnects - "your connection was closed due to an error"

I am attempting to deploy OpenFire with FastPath and Spark.  I have about 25 internal users; in my current test build I'm running with only about ten.

All users regularly see the "your connection was closed" error.  Some users will ALWAYS see it upon messaging another specific user, while messaging others does not cause the error.  Some users see it immediately upon messaging anyone.  It's vexing.

I'm using the newest builds of OpenFire and Spark.  My network is robust, the servers (Win2012 Datacenter on VMware) are local.  DB server is MySQL 5.6.  User authentication is local, not via AD.

OF logs attached.  I'm pretty stuck here, and I'd be most grateful for a fix.  Thank you in advance for any help any of you might be able to contribute.

-Scott