• Minecraft Version: 1.12
• BetterFps Version: 1.4.6
• Forge/LiteLoader Version (if any):
• Crash Logs (if any):
• Other Mods (if any): optifine

Logs?

I have the same issue - also BetterFps-1.4.6.jar in MC 1.12.
The server provides no crash-report, therefore I am unable to share any useful logs.

EDIT:
@Guichaguri Sorry about the lack of info.
Running Forge 14.21.1.2424 and these mods.
The server can only start up with BetterFps-1.4.6.jar removed.

@Sinuce Are you using Forge? LiteLoader? Any other mods?

@Sinuce does it only happens server-side?

@Guichaguri Yes. Only server-side.

UPDATE:
Please don't spend time for my sake working on this. ^^;
I have simply removed BetterFPS from my server.
I merely wanted to provide you with version data.