As you can see this log doesn't really tell you much. To track down the culprit I loaded everything up in intellij and threw a breakpoint on the line that throws that NBT error in the vanilla code.
The following image is what Intellij's debug stack throws back when the disconnect happens:
This disconnect happens seemingly at random even when no one is using Ars Magica stuff.
Version: ArsMagica2-1.10.2-1.5.0-17b.jar Client log: http://pastebin.com/LYf13FY5
As you can see this log doesn't really tell you much. To track down the culprit I loaded everything up in intellij and threw a breakpoint on the line that throws that NBT error in the vanilla code.
The following image is what Intellij's debug stack throws back when the disconnect happens:
This disconnect happens seemingly at random even when no one is using Ars Magica stuff.