OptiFine is installed on a dedicated server
Applies to Forge, NeoForge.
What it looks like in your log
java.lang.ClassNotFoundException: optifine.OptiFine
What causes it
OptiFine is a client-only mod. On a dedicated server it crashes on load or quietly breaks other mods.
How to fix it
Delete the OptiFine jar from the server’s mods folder. Players keep it in their own clients.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log