Address already in use / Failed to bind to port / EADDRINUSE means something inside your container already listens on that port. It is never another customer, because containers are isolated.
Cause 1: a previous instance is still running
The server was killed but a child process (a Java process after a bad restart, a Rust/Unity process that ignored the stop) is still alive. Fix: Kill from the panel, wait 10 seconds, Start. If it happens after every restart, tell us in a ticket; we look at the container.
Cause 2: two things want the same port
A plugin or mod configured to the game port: Dynmap's web server, a voice chat mod, enable-query with query.port equal to server-port, Geyser on the Java port. Give it its own extra port or disable it.
Cause 3: a config hard-codes a port you do not own
server-port=25565 written by a plugin or restored from another host. The panel resets server-port on every start for Minecraft; for other games check the config file against the Network tab.