This morning, many users noticed an outage where they were unable to download a number of libraries when installing a modpack, resulting in crashes when the modpack tried to launch. We just rolled out Launcher build Stable #374 to address this issue. So what happened?
Surprisingly, it wasn't a bug in Stable #373 as many people thought- this morning, Mojang rolled out a change to move the core Minecraft libraries to a new location, leaving us unable to download them. The change we rolled out pointed the launcher correctly at the new location.
So how can we prevent this from happening in the future? We've received a few resources that will help us stay better apprised of what Mojang is up to, ideally allowing us to learn of changes like these BEFORE they go live. We hope this will allow us to better serve you in the future, with fewer outages like these. Thanks so much for your patience and support!