This definitely seems to be an issue with whatever repository is being used to host these libraries.
Here are the logs for successive runs:
1: https://gist.github.com/anonymous/f2bf14835e7346645d2082ccd6758b6f
2: https://gist.github.com/anonymous/58663a58fbbcd3223ef7fc45aa36b054
3: https://gist.github.com/anonymous/8e682d0a9f5fa65a10a928b6393f7ea5
Seems like requests for downloading the libraries are randomly dropped. Some new libraries are downloaded on successive runs, and if some libraries are still missing, it fails. This error won't occur if installing to a directory that already has these libraries installed.
I've tested this on a Windows machine running Java 8 (source of the logs), as well on a remote server. Both machines had no firewall active when testing the installer. Also tested it with the JVM arguments you had listed above, and had the same issue. Both scenarios were installing to empty directories.
On the remote server, forge was being installed in a containerized set up (also with Java 8), where just two days ago it would run fine. The image and configuration for this container did not change since then, and now encounters this same problem.
