What is the status code of that page? Is it actually an empty response, or does it just look blank in a browser?

When you access it from the internet, do you use an ip and port or a domain name? My first guess is that you entered the domain name somewhere in the config of either Opencloud or a reverse proxy, and the mismatch (between the expected domain name and the ip address used instead) is what causes trouble.

because /etc/hostname is allowed to contain only ASCII Latin characters, numbers, and dash

If you really wanna go for cursed, the syscall to change hostname doesnt have this restriction, and I do have a working system with non-ascii characters in the hostname. It takes some fighting with systemd and NetworkManager, and stuff does sometimes break, but it can be done.