I don't know if I am late but I had to deal with a problem.
The servers for Forge for 1.12.2 would not download. It was the e-tag/jar problem, all I did was find the 1.12.2 server jar file (link below).
I put the jar file in a folder and put the server download directory into where the jar file was (make sure to download it more then once because at first mine had an error)
This is for all versions (ig) because they are all the same error.
Hope this fixes your problems!
https://s3.amazonaws.com/Minecraft.Download/versions/1.10/minecraft_server.1.10.jar