As pointed out in this answer by Thomas over at AskUbuntu, it's easy enough to stop or disable this server, but you may want to consider whether you actually might want it running before you do. I found that question while researching this exact same concern on my own Kubuntu 18.04 LTS machine, and it helped me dig deeper into what it actually was. Turns out it's used to serve
monodoc-http, as I was able to determine by exploring the content of files found in
If you installed MonoDevelop or MonoDoc or anything related to developing code in Mono, then that's where it came from. If you did that on purpose because you're learning or coding in Mono, then you may actually want that service running. If you have no use for MonoDoc services, you can easily disable them on flavors of Ubuntu using
sudo systemctl stop mono-xsp4.service will stop the service
(but won't disable it, so it will run again after next reboot).
sudo systemctl disable mono-xsp4.service will disable the service
(but will not stop it if it is running currently).
You'll need to run both commands if you wish to actually stop the service now, and disable the service so that it's not running after the next reboot. If you know for certain that you have no use at all for MonoDoc and related services, you can also search for them in your package manager and uninstall them entirely.
As to why it's throwing an exception, that I do not know. It does the same on my system if visited directly in a browser. My best guess on that is that it probably requires parameters passed in which MonoDoc would hand it by default that just visiting it directly in a browser do not give? (Anyone who knows for sure is welcome to edit this answer to include that information, or contact me with that info so that I may update this.)
Note: This is a nearly exact copy/paste of my answer to the same AskUbuntu question that I linked here, as it also happens to be an accurate answer to this question here as well...