Id mismatch error with hexxit

Did you also install the modpack onto your client?

Also make sure that you've installed the same version of the modpack on your client.

Which Launcher do you use?

Which modpack version and which minecraft version is your client?

Please try to reinstall the client and the server.

Please note that your server will be resetted then.

Why don't you play on english?

no.

You only have access to the plugins folder and the world.

Check the Guide which Julian posted