When I turn on my server, it crashes and won't let me open it

it looks like the forge mod loader mod (fml) is outdated or is installed as the wrong version of forge

hope this helps

If your server crashing And Can't Even Start

Then 1. Go To Plugins/mods folder and see every mods config.yml it's important because some plugins might give problems/errors.

2. If step 1 doesn't solve the Problem Then Try to Uninstall All Plugins Or Mods and Start Server then later Do Install All.

3. If now also Showing Errors then last option is Deleting All Plugins/mods and installing One By One

After Installing One Restart Your Server like this you can know Which Plugin/mod giving Error

I know This Steps are difficult mostly Last one but it may resolve your problem.

Thank You,

PG