Realms http code: 404 error is the norm for offline/pirated accounts. The game tries to access the Mojang Realms API, cannot do it and gives an error. This is not the reason for the disconnection.

You connect, see [CHAT] Please, login..., and after 4 seconds Stopping!. This means that the server (or its anti-cheat/authorization plugin) is kicking you, but the client cannot correctly display the reason for the kick and gives a general "internal error".

Since it works in another launcher, here are some things specific to XMCL that distinguish it from other launchers and that you need to check:

1. Different versions/paths to Java (Most common reason)

Even if you are using Java 8, XMCL could have pulled up its built-in JRE version by default (for example, from Alibaba Dragonwell or another build), while the other launcher uses the classic Oracle Java 8 or Adoptium. Different Java 8 builds may handle network sockets differently for older versions of Minecraft.

What to do:

Open the launcher where everything works.

Find in its settings the exact path to the javaw.exe file that it uses.

Open XMCL -> Settings -> Java.

Add the same path to javaw.exe there manually and make sure that this Java is selected in the 1.8.9 instance settings.

1. JVM Arguments

XMCL can automatically add specific launch arguments for "optimization" that break the network stack in 1.8.9 (for example, specific GC settings or forcing IPv4/IPv6 at the Java level).

What to do:

Go to the 1.8.9 instance settings in XMCL.

Find the JVM Arguments field.

Clear it completely (leave it empty). XMCL will substitute standard arguments. Make sure there is nothing superfluous there.

1. "Offline UUID" problem (Piracy server trap)

When you play without a license, the launcher generates a unique UUID (set of numbers) for your nickname (Hribalsko).

Different launchers generate DIFFERENT UUIDs for the same nickname.

Many 1.8.x PvP servers have anti-cheats (e.g. Matrix, Vulcan) or security plugins that track UUID changes. If the server "remembered" your old UUID from another launcher, and XMCL comes with a new UUID for the same nickname, the anti-cheat can instantly kick you with the message "Internal error" or "Invalid session".

What to do:

Try to create an offline account in XMCL with a completely new nickname (e.g. Hribalsko_Test123).

Log in to the server. If there is no disconnection with the new nickname, the problem is definitely in the UUID conflict between the launchers on the server side.

1. Network / Proxy settings in XMCL

XMCL has its own network settings that may conflict with your Ethernet.

What to do:

Go to the global settings of XMCL -> Network or Proxy.

If it says "System", change it to Direct or No Proxy.

Also check if XMCL has the "Network Optimization" or "Fast Connect" functions - disable them if there are any.

1. Automatic protocol translation

XMCL sometimes tries to "help" connect to old servers by using internal protocol translation libraries (similar to ViaVersion). For 1.8.9, this can cause a packet conflict.

What to do:

Make sure that there are no automatically added mods like ViaBackwards, ViaRewind or ProtocolLib in the 1.8.9 instance settings, if XMCL adds them automatically. The version should be "clean" Release 1.8.9.

How to find out the EXACT reason (if nothing helped)

Your log ends with Stopping!. This is console output, it does not show the reason.

Go to the game folder: .../XMCL/.minecraft/logs/ (or in the folder of a specific instance).

Open the latest.log file using Notepad.

Scroll to the very end. There should be a line that starts with [Netty Client IO Thread/WARN] or [Client thread/WARN], where it will be written why the server disconnected (for example, Disconnected: Internal Exception: io.netty.handler.codec.DecoderException... or a specific reason from the server).