Not entirely sure who's to blame for this crash, but when breaking a containment jar with a captured mob such as a blaze or camel, or an openblocks trophies such as the wither trophy, the game will run into a crash if you also have the mod neat installed.

In both cases disabling neat stopped the crashes from happening.

Neat + Ars-nouveau Crash log: https://pste.me/@1xvU6C1e7ss
Neat + Openblocks trophies: https://pste.me/@XGZqU-eIG3U

FTBTeam/FTB-Modpack-Issues#5808
FTBTeam/FTB-Modpack-Issues#5790
FTBTeam/FTB-Modpack-Issues#5846

Thanks for reporting this. I will take a look at it today!

This commit: e7e52b7 should fix this. I will close this issue when I release the update in a few days