Can't start server

I have confirmed my server so that it should start, but now it is just stuck showing starting and never actually starts.

I have chosen a modpack and changed my settings to what I need, but the server is still stuck on the "starting" phase.

Where do I find the log?

I am trying to play Modern Skyblock 2

My server started on the second try, but I can't connect to it.

It spends about two minutes trying to log in, only to be disconnected.

I can try to connect twice before the server is closed again.

I still can't connect.

I tried using DynIP but can't connect with that either.

After about a minute or so, it says "Connection lost: Disconnected".

I can connect to other servers fine, so I don't know what the problem is.