I have been playing with my friends linked through hamachi but every once in a while it shows this and crashes everyone but the server stays up and can be rejoined afterward. It is quite often though and is game-breaking.

 

This shows before everyone crashes.

 

[21:57:43] [Netty Server IO #4/ERROR] [FML]: NetworkDispatcher exception
java.io.IOException: An existing connection was forcibly closed by the remote host
        at sun.nio.ch.SocketDispatcher.read0(Native Method) ~[?:1.8.0_201]
        at sun.nio.ch.SocketDispatcher.read(Unknown Source) ~[?:1.8.0_201]
        at sun.nio.ch.IOUtil.readIntoNativeBuffer(Unknown Source) ~[?:1.8.0_201]
        at sun.nio.ch.IOUtil.read(Unknown Source) ~[?:1.8.0_201]
        at sun.nio.ch.SocketChannelImpl.read(Unknown Source) ~[?:1.8.0_201]
        at io.netty.buffer.PooledUnsafeDirectByteBuf.setBytes(PooledUnsafeDirectByteBuf.java:288) ~[PooledUnsafeDirectByteBuf.class:?]
        at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1100) ~[AbstractByteBuf.class:?]
        at io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:372) ~[NioSocketChannel.class:?]
        at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:123) [AbstractNioByteChannel$NioByteUnsafe.class:?]
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:624) [NioEventLoop.class:?]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:559) [NioEventLoop.class:?]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:476) [NioEventLoop.class:?]
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:438) [NioEventLoop.class:?]
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858) [SingleThreadEventExecutor$5.class:?]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_201]

 

 

Here is a Drive Link to the Crash and Images of the problem. https://drive.google.com/drive/folders/1Ei07XrK5VSifhrmBsC4mvIdbMBbWGu1C?usp=sharing

 

Quote

WARNING: coremods are present:
  HCASM (HammerCore-1.12.2-2.0.4.7.jar)
  LoadingPlugin (HardcoreDarkness-MC1.12.2-2.0.jar)
Contact their authors BEFORE contacting forge

Looking through the crashlog, the only mod mentioned in any of the errors/waiting threads is hammercore. Not sure if that's the issue, but if it were my problem, I would start by removing that, and seeing if it still crashes.