OK, so it appears something was wrong with the environment hosting my docker container (a CentOS7 VM). I rebuilt the same exact environment on an FC24 (non-VM) machine (newer docker, same containers, etc.) and the exception just disappeared.
Lessons learned:
- The dream of liberating oneself from environment dependencies through containerisation is, on the face of it, a delusion. Ghost bugs/execptions can - and do - appear due to the container host environment, without telling you much about it, and being therefore more elusive than ever.
- Logstash (well gork's) exception logging is less than ideal. Whatever was causing the original exception (if I had to bet, I'd bet file system and/or SELinux problems, but I really still don't know) was most emphatically not a pattern problem
Thanks to all who bothered to (even) read wasted time on this.