Can you post the log for a session where this happened? MFBridge does have a catch for all mobs that try to drop leather drop hide instead, so if something is making them drop leather the log will have a bunch of messages about it.
-Fixed the extended crash protection so it doesn't cause mismatch errors
-Disabled some drop handling when MF 1.3.3+ is installed
I tried updating Java a while ago but completely removed 8 back then. I did find out about that issue when I searched for the error, but everything I found from the search talks about not even being able to start a game, whereas I was playing a game and then it just crashed. Not really sure.
And apparently I have legacyjavafixer anyway as default in the modpack.
Also, I found a crippling recipe incompatibility between MF and Thaumcraft - the Alchemical Furnace requires a vanilla furnace, which at least with MF hardcore options on, is of course uncraftable...! Can't even replace it with an MF Furnace Heater with MineTweaker since of course it's Thaumcraft.
Hey, how do I make a dagger of sacrifice? I've tried iron and ornate swords, and an ornate dagger. The Bloodmagic wiki says it's an iron sword, so I figured you might've kept that, but I don't want to keep wasting materials trying to figure it out.