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!
@Spreewald1 I love link rot :D
I found a link to a maven repo that has those exact files you need (from my old 4.1.27 comment) here: https://repo1.maven.org/maven2/io/netty/netty-all/4.1.27.Final/
Download netty-all-4.1.27.Final.jar and follow the instructions as normal; I verified they are the complete same with file sizes and last modification date