HI, so I started up a core on my server. The next thing I know, when I try to join my server, it says 0/-4 people online. So I go and try to join, and it says Server Is Full. I'm so confused. What might have gone wrong?
check properties file of the server. If everything is set up correctly, check in with your core to see if there are any code that sets the max players to that.
What core are you using exactly? And if it's one you made yourself can you please tell us about it? It appears that it has somehow modified your max players to a negative number hence why nobody can join. As far as the server is concerned the number of people online is four over the limit.