Originally reported as McJtyMods/XNet#228. When we call getStackInSlot, an AbstractMethodError results. At that location, handler is an instance of SonarInventory (it came from a TileEntityCalculator.Atomic if that's relevant). It looks like AbstractSonarInventory, which SonarInventory inherits from, does indeed implement this method, so I'm not sure what's causing this.

downgrading to earlier version works without crash, but i had to downgrade calculator and flux network otherwise it crash.

I'm getting a similar crash. World will crash on load. Here is the multiMC instance containing the two worlds, pre has setup for crash, and post where world is corrupt.
https://www.dropbox.com/s/n1ogcyvoa4dkrcn/Calculator%20Crash%20Repro.zip?dl=0

Crash log
https://cdn.discordapp.com/attachments/323830072996003840/414913647488401409/crash-2018-02-18_14.35.27-server.txt

Forgot to close this a while back, was caused by a IItemHandler and IInventory being implemented on the same class, meaning their subtly different getStackInSlot methods clashed.