Read the Extra Bits at the bottom to find the real reason the server crashes.

Expectation:
Start Minecraft server; Log in; Have fun!

Reality:
Start Minecraft server; After an amount of time (usually within 2 minutes, sometimes immediately) the server software will crash.

Reproduction of the bug:
1) Start up a Minecraft 1.8.1 server (In my case, it was upgraded from an existing 1.8 server).
2) Wait a little while. Usually 2 minutes or less.
3) The server software will crash with the error "Ticking Entity."
HOWEVER, I've found this problem to be exclusive to me (or, presumptuously, others with spawn chunks like mine)

Extra bits that may be helpful:
Looking at the crash report, it appears the problem is caused by a poppy. There is a redstone machine that shoots a poppy into a Nether portal about every 40 seconds. I have not been able to reproduce this problem on any world except my own, however I can provide the necessary chunks of my world to recreate the problem. The problem occurs with every backup I have that contains the machine I described.