I came back here to see if anyone else had the same problem, seems like the server for the leaderboards goes down at least once a week. At the time of writing this comment it's been down for nearly 3 days so far.
Yeah, at least once a week, usually it's during the weekends (in my timezone) so I assume it was just weekly maintenance, but in this case I assume the servers crashed and the developer didn't know. I should probably send them an email about it or talk to them on discord.
edit: just now sent him an email, so fingers crossed!