Get your minecraft_server.1.12.2.jar
from the root of your server, download the last bundled jetty 4.1 version https://repo1.maven.org/maven2/io/netty/netty-all/4.1.68.Final/netty-all-4.1.68.Final.jar
Open your server jar with 7zip and navigate to minecraft_server.1.12.2.jar\io\netty\
open the netty jar file you downloaded and go to netty-all-4.1.68.Final.jar\io\netty\
back in the minecraft server jar delete these 6 folders
now simple copy the same 6 folders from the netty package into the minecraft jar
throw the jar back into the root of your server and you are done!
The link with the netty download does not work for me. It takes me to a blank page which only says "Forbidden".
Is there any other link I can use to get the right netty version. I tried downloading it from https://github.com/netty/netty/releases/tag/netty-4.1.37.Final but there I was not able to locate the right file I needed.