Deth Invictus

Real world funguses generally don't grow on these, either.  LOL.

I wouldn't mind looking at your r.0.0.mca file from your world's region folder. I have had to fix a similar issue on my snapshot servers with pillagers riding beasts where the beast is killed from under the rider which results in the same error. I'd be interested to see if it is the same problem or something else. I can probably fix it for you.