: Ensure your base client jar and the Rejects addon build are explicitly compiled for version 1.21.1. Mixing variations designed for minor sub-patches like 1.21.7 or 1.21.11 triggers instant class loading errors.

In this long-form guide, we will dissect every possible meaning of explore the root causes, and provide step-by-step solutions to resolve the issue permanently.

Go directly to the official AntiCope GitHub Repositories to download the latest compiled 1.21.1 .jar file.

Make sure you have downloaded the correct installer for your specific game version.