How much RAM do you have available on your PC? Because the minecraft server takes up 4GB of your computers RAM, the modpack is for what I believe also a very heavy modpack which will require around 4GB or more RAM for your client aswell.
If you only have 8GB of RAM your game will crash because it is out of RAM. If you have 16GB of RAM you should check how much the game and/or server is actually using.
If you lower the amount of RAM of the server, it will probably become very laggy and unstable.
If you have any more questions, do let me know!