In my plugin I'm using settings.getInt(["server", "port"]) to obtain the server port. However, I found out that when OctoPrint is launched with octoprint --port=5001, the value returned from settings.getInt(["server", "port"]) is still 5000.
What's the right way to get the port OctoPrint server is actually listening on inside a plugin?
I suppose @foosel would like to have your octoprint.log file.
I think I'd also wonder if you are trying to connect via IPv4 or IPv6. From my own naïve viewing of this code, it looks like it's hard-coded to 5000 but there's probably other stuff going on that's lost on me.
octoprint.log files doesn't show anything since nothing is being logged.
She's in Germany, btw. If she does stop in, it will be hours from now.
Hi!
Keep in mind: Today is National Holiday.
I found the answer myself. It's actually documented here: http://docs.octoprint.org/en/master/plugins/mixins.html#octoprint.plugin.StartupPlugin.on_startup
But I wouldn't have found the document if I hadn't had traced the code all the way there. That's why software is so hard isn't it. 
Actually, I love open-source software. Many times people ask questions about OctoPrint or a plugin and I can just read through the code and sound authoritative, as if I actually knew what I were talking about. ha
Just back from vacation, but I see you already found your answer 
