Some players experience lag and disconnect randomly

Hey everyone, I have a seemingly impossible problem - let's see if someone here can help me.

I host a Fabric Minecraft server on my VPS using CraftyController, and every few hours some of the players (who has the problem differs every time) experience a lot of lag and sometimes get kicked. The connection is stable otherwise.

The mods used on the server are attached as a screenshot.

However, the server seems to still be running fine and CPU, RAM and TPS are all okay. There also isn't a real error in the console, only sometimes after a while it throws:
[21:22:00] [Netty Epoll Server IO #4/WARN]: Failed to deliver packet, sending fallback clientbound/minecraft:system_chat
io.netty.channel.StacklessClosedChannelException: null
    at io.netty.channel.AbstractChannel.close(ChannelPromise)(Unknown Source) ~[netty-transport-4.1.118.Final.jar:?]


On a client which is experiencing the problem it throws:
...
[21:20:18] [Netty Client IO #5] [Essential Logger/ERROR]: Network Exception Caught
java.net.SocketException: Connection reset
    at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(Unknown Source)
    at java.base/sun.nio.ch.SocketChannelImpl.read(Unknown Source)
    at knot//io.netty.buffer.PooledByteBuf.setBytes(PooledByteBuf.java:255)
    at knot//io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1132)
    at knot//io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:356)
    at knot//io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:151)
    at knot//io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:796)
    at knot//io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:732)
    at knot//io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:658)
    at knot//io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
    at knot//io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:998)
    at knot//io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at java.base/java.lang.Thread.run(Unknown Source)
[21:20:18] [Netty Client IO #5] [Essential Logger/ERROR]: Network Exception Caught
[21:20:18] [Netty Client IO #5] [net.minecraft.class_128/ERROR]: Negative index in crash report handler (13/21)
...


We also noticed that if you move a lot in that moment, you're more likely to get disconnected.

I suspect this has to do with the connection to server, and it's caused by the provider - or am I wrong?
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