We are currently experiencing issues with Mojang’s authentication API (authserver.mojang.com), which seems to affect the installation of Forge-based modpacks. If you are having trouble installing or launching your favorite modpacks, this could be the reason.
We are closely monitoring the situation and hope for a swift resolution from Mojang. In the meantime, we appreciate your patience and suggest regularly checking for updates regarding this issue.
Update:
The situation has somewhat evolved today. It appears that Forge has implemented a patch to address the issue. However, modpacks that use Forge must now deploy a new version of their modpack to be compatible with the latest version of Forge. Until then, all modpacks using the unpatched version of Forge (which is the majority of modpacks), will no longer be able to be installed!