While this is a spigot server, the stack trace contains nothing but minecraft classes (except for the HashMap class). This issue occurs not only with furnaces as the block but some have been on hoppers, and one was just  Description: Exception ticking world

One of the crash reports can be found at:

 https://gist.github.com/blarosen95/f2072d8413d1dba4b0525bc86ce4db55