The pack hosts itself

• New: MC GUI uploads its resource pack to ResourcePack AI's free pack host by default (resource-pack.mode: upload), so players download it with nothing to open or set up, on any host. It is only uploaded again when it changes, and only the Minecraft client can download it. If the upload fails, MC GUI serves the pack itself until it works.
• Upgrading: your config.yml keeps mode: serve. Set mode: upload and restart, unless you host the pack yourself on purpose.
• New: with mode: serve and public-url blank, the console warns at startup that only a player on the server's own machine can download the pack.