When I deactivate the mod, everything works fine.
The error appears in the Nether while we fight against some Wither skeletons, more than that we don't know how to reproduce the problem
For now I disabled the mod, but I would like to incorporate this beautiful mod

https://pastebin.com/tGvYhQj4

Woah, that is a major bug! I can't believe I missed this. Was able to duplicate it locally, and just merged in a fix for it. Expect an updated version of the mod available sometime later today (0.2.3-1.19.2)