[MC-55876] Bad packet id 50 / java.io.IOException

Client Error:
Internal Exception: java.io.IOException: An established connection was aborted by the software in your host machine

Server Error:

25.05 09:36:11 [Server] INFO /192.168.1.100:57056 lost connection
25.05 09:36:11 [Server] INFO Disconnecting /192.168.1.100:57056: Protocol error
25.05 09:36:11 [Server] INFO at cj.run(SourceFile:94)
25.05 09:36:11 [Server] INFO at ci.c(SourceFile:16)
25.05 09:36:11 [Server] INFO at ci.i(SourceFile:250)
25.05 09:36:11 [Server] INFO at ei.a(SourceFile:193)
25.05 09:36:11 [Server] INFO java.io.IOException: Bad packet id 50

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This seems to be happening even when I switch to different JAR files, like spigot and craftbukkit.

Network and Firewall are set up correctly.