If you are getting a ClassNotFoundException on a modded server, it usually means you are missing a mod dependency.

How to Fix This

1. Read the error carefully - what class is it failing to resolve? The class name usually tells you which mod you need to install.
2. Check mod dependencies - read the list of dependencies for any recently installed mods. Do they require a library or API mod you haven’t installed?
3. Install the missing mod - download the required dependency from Modrinth  or CurseForge  and upload it to your mods folder.
4. Restart your server after installing the missing dependency.