A mod or plugin that crashes the server is usually built for a different Minecraft version, for a different loader (Forge instead of Fabric), or is missing a dependency.
Remove the last mod or plugin you added (Uninstall in the Installed tab of the Plugin Manager, or delete its file in
mods) and restart to confirm.Check its page: Minecraft version, loader, required dependencies.
"Client only" mods (minimap, shaders): they never go on the server.
Add mods one by one to isolate the one causing the issue.
Common errors
- "Missing or unsupported mandatory dependencies" (Forge, NeoForge) or "Incompatible mods found!" (Fabric): the console names the mod at fault and the dependency or version it expects.
- A mod made for the game client only (minimap, shaders) crashes the server on startup, with an error mentioning
net/minecraft/client: remove it from themodsfolder.