I'm definitely not using it for anything else. I haven't done much with this server, except set up Plex (port 32400), some Time Machine backups, and repeatedly tried installing Deluge, Sonarr, Radarr, Lidarr, and Jackett. Currently, I only have Deluge, Jackett and Sonarr running, and none of them use that port (although I am having trouble getting Sonarr to connect to Deluge).
Would changing the default port be as simple as editing the Docker settings? At this point, I'm very leery of doing any step I don't fully understand, due to all the troubles I've been having with it all.
Edit: Okay, I've changed the port as described above, and it works now. However, there shouldn't be anything else assigned to that port, and I'd still like to find a way to free all previously assigned ports that are no longer in use, and otherwise clean up the system. Changing the default port when no other port should be using it feels like a workaround, not a fix -- and figuring it out should help me learn this system better.