Versions

Betweenlands version: 3.4.1

Forge version: 14.23.5.2772

Singleplayer or Multiplayer:

Multiplayer, Sponge version: 2768-7.1.5-RC3506

Installed mods (server):

Antique Atlas, AppleCore, AutoRegLib, Carry On, Cases, ContainerFix, CraftTweaker JEI Support, CraftTweaker2, Earthworks, Example Mod, Fast Leaf Decay, JurassiCraft, Just Enough Items, LLibrary, Ore Veins, Quark, Rough Mobs 2, Rustic, Spice of Life: Carrot Edition, SpongeForge, The Betweenlands, Wearable Backpacks

Link to full crash log

https://gist.github.com/pingvikin/9a05ef5e262dab6f17a2661cafadb5a4

Description of the problem

How to reproduce:

1. Player joined in the Between world
2. Server chashed

Some console spam:

https://gist.github.com/pingvikin/f66313bd66a9cec8e29fe96bbe71a6f3

Hm, that looks like a SpongeForge problem, please report it there: https://github.com/SpongePowered/SpongeForge/issues.
Thanks.