I tried getting polytone on my modded world and it crashed without me even moving. THEN i tried opening up a world only with polytone, no texture packs, and i tried stuff i tought would crash it. I tried spawning horses and putting armor/saddles, tried a ghastling, and THEN the ideea struck me. LEASHES ARE THE PROBLEM!! So, when leashing any kind of mob, passive or aggresive, would instantly crash the game.
(first log file is from my modded world, second is from only trying with polytone alone)