Actually, just checking and see that Discourse ID already works on my site now. As I said, I restored my site so this is no longer a problem for this site.
But I am curious to see what would happen if I repeated the steps to create a new site which I will do as soon as hetzner lets me! Maybe after I pay them for my first invoice next week on 1 February.
Could this have been related to the fact that I changed the IP for the domain at namecheap not long before spinning up the site? Maybe the domain wasn’t resolving to the new IP for everybody yet.