Issue Description:

Following appears in log
https://pastebin.com/raw/6YwtEvkM

What you expected to happen:

No errors in the log :')

Steps to reproduce:

Unknown at this point.
...

Affected Versions (Do not use "latest"):

• EnderIO: EnderIO-1.12.2-5.0.50.jar
• EnderCore: EnderCore-1.12.2-0.5.65.jar
• Minecraft: 1.12.2
• Forge: forge-1.12.2-14.23.5.2838-universal.jar
• SpongeForge? spongeforge-1.12.2-2838-7.1.7-RC3886.jar
• Optifine? no
• Single Player and/or Server? Server

Your most recent log file where the issue was present:

https://pastebin.com/raw/6YwtEvkM

Reproduce on latest version.

I blame spongeforge

That's a bug in whatever block you are harvesting. They are dropping an unmodifiable list of items, which is not valid.