They are already aware of this and are working on it, they gave manual steps to fix in #dev-updates
NeoForge versions fixed
We (along with the NeoForge team) have fixed the metadata issue that caused versions of NeoForge to fail to install. Unfortunately, users who attempted to install NeoForge while it was broken may have issues installing it now.
We're working on an update to automatically fix it for you, but if you want to fix it manually right now, here's what you have to do:
Manual fix steps
- Open your
ModrinthApp data folder. An easy way to access this is to open the folder for one of your instances, and then go up to the ModrinthApp folder a few levels up from here.
- Go into
/meta/versions and find the version that you're having issues with. It's likely 1.21.1-21.1.something. Delete this folder.
- Close and reopen Modrinth App, and it should now work.