Forge log link: http://pastebin.com/EGMG7pUa (Crash Log Error. Whole Forge Log too large for free users)
Log File: latest.zip

Problem: Crash on trying to use Spirit Furnace to summon a mob

Expected behaviour: Intended operation of summoning the mob and providing the drops

Steps to reproduce the problem: (In Creative, but should occur in survival as well I would imagine)

• Kill a mob
• Use a Vengeance Focus to capture the spirit with a Box of Eternal Closure
• Recover the now filled Box of Eternal Closure (Either by Creative "Pick Block" or Survival pickaxe)
• Build a Spirit Furnace of appropriate size
• Fill the furnace with 10 buckets of blood
• Place the Box of Eternal Closure with the spirit in it into the Spirit Furnace
• Wait for the progress bar to fill
• Complete game crash upon reaching 100% progress

I think I already fixed this in the latest 1.9.4 version (I'll check later to make sure)
I won't be doing any new releases for 1.9 I'm afraid.

This seems to be fixed in 1.9.4 - I can't reproduce it and like mentioned above, there won't be any future updates for 1.9.x < 1.9.4