The network layer was shut down while the server was still running
What it looks like in your log
java.util.concurrent.RejectedExecutionException: event executor terminated
What causes it
Netty's event loop, which carries every player connection, had already been shut down when the tick loop tried to use it. That happens when a plugin or a /reload tears down the network stack, or after the server hung and was told to stop while ticks were still running.
How to fix it
Restart; it does not recover on its own. Then stop using /reload. If it repeats without a reload, the plugin that touches networking (ViaVersion, ProtocolLib, a packet-level anti-cheat, a proxy bridge) is the one to update or remove, and check the lines before it for a hang that started it.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log