Since the official Sigma servers are offline, the standard client often gets stuck on the loading screen. Community Fixes

We have fixed many features already, Animations being broken. Music player. Added back viaversion from 1.7 all the way up to 1.21. ImGGAAVVIINN/Sigma-Client-5.0-Archived - GitHub

has fixed broken animations, music player functionality, and updated version support (viaversion) to allow it to run on newer Minecraft versions like 1.21.1