Can you show me the full container and blockentity classes?
I haven't worked with this very much/at all but since I am working on a mod that requires a custom recipe slot things, I might be able to help.
It seems that the problem is with the "Wither Sickness" effect. The only solution I can think of is to disable it in the config and use an nbt editor to remove the effect manually.
A: We don't accept reports in Engrish.
B: 1.8.9 has been out of support from this forum since before the dinosaurs some long time before I joined here (probably some time in 2015-2016).
You are running a hybrid server (Magma). Plugins and mods are not intended to run together so of course you are going to run into problems.
There are plenty of alternatives to various plugins like EssentialsX (FTB Essentials) and Luckperms (Forge Ver.).