How did you spin up the self-hosted instance?
Seems the self-hosting documentation is a bit... lacking. Both "self-hosting" buttons on the main page send me to "#".
It seems interesting, even though I'm more of the roll your own school of thought here because there could be some very sensitive data in this system.
Here you go: https://docs.monicahq.com/developers/setup-local-development
That's linked from their GitHub: https://github.com/monicahq/monica
PikaPods also offers Monica hosting in your own container.