zorowinegar

When you update your modpack, but don't increase the modpack's version, the launcher's cache doesn't get reset. That means that it keeps using the old download link for up to about an hour. If you increase the modpack's version, a simple launcher restart is usually enough to trigger it. Downloaded fine on my end now though (probably because its 12 hours later now).

The Forge installer, renamed to modpack.jar, just goes into the bin folder. You probably included the universal forge, but that one doesn't work for the last few versions of Forge for 1.12.2.