Hi Taco,
The error you're encountering seems to be related to issues with the ATLauncher's ability to fetch and load versions of Forge. The error message you provided indicates a "HTTP 503 Service Unavailable" error. This error typically means that the server you're trying to access is currently unavailable.
If the problem continues, you might want to explore alternative Minecraft launchers, such as the official Minecraft Launcher or other third-party options. Sometimes, a different launcher can bypass issues with one specific launcher.
Remember to back up your saved worlds and configurations before attempting any major changes to your launcher. This will ensure that you don't lose your progress in Minecraft.
Have a great day!