[ Please replace text in [] with the answer from the above question! ]

Do you use Java 8?
yes

What version of Minecraft do you use?
1.12.2

What version of MinecraftForge do you use?
1.12.2 14.23.2.2635, server is universal version

What version of SanLib do you use?
SanLib-1.12-1.3.1

What version of Clay Soldiers do you use?
ClaySoldiersMod-1.12.2-3.0.0-alpha.9.1

What is the issue / bug you have encountered? Give us a short description.
Server crashes when playing on clay soldiers sometimes.
How can it be reproduced?
I gave a powder block to a diamond cape soldier and went into a cave and placed torches, but I backuped the server and still does it so I'm not sure.
crash-2018-03-28_08.17.03-server.txt

Please post your log file (either in /logs/fml-client-latest.log or /logs/latest.log) on https://gist.github.com and provide the link:
https://gist.github.com/BobRossss/f959a4df0ee9001b9d3a738034235fbc

uh... so your java apparently doesn't define Arrays.asList() as usual like in the JDK and doesn't return a modifiable list (edit: apparently this is default behavior to have a immutable list returned and the JDK does it differently for whatever reason)...
I'll try to fix it in the next version.

Thanks for quick response! Glad that this is gonna be fixed.

Forgot to mention, this bug has been fixed in the commit bcbb56b
It will be public once the Clay Lexicon has been finished.