Appears to be from this mod, or maybe something else doing somethint to it. Crashing occurs in nether, probably happen in oveworld/anywhere though

https://gist.github.com/ShootToKillSTK/9a6e0475e86fb6b0b600af587f909521

Got a similar, but different, crash: https://gist.github.com/belathus/569038881f4a655c3e5321597cf194f0

Quick glance tells me that the Skeleton Vanguard is having some compatibility issues with its shield usage AI, I'll bring this up with the team

Fixed in dev, will be fixed in next release