Updown supports IDN fine, this website is simply:
- Not reachable under IPv6 despite offering one in the DNS
- Returning 400 to HEAD requests on IPv4 (or 503 to GET requests, which is a violation of the HTTP spec BTW)

You can check your downtime details: https://updown.io/downtimes/69dd339823f2a319dd3c2757/results/69dd339823f2a319dd3c2753

So if it's yours and you want to monitor it properly you'll have to:
- Fix the IPv6 connectivity or remove the broken IPv6 from DNS. Alternatively if it's not yours, I can manually disable dual check on this one.
- Use the contains feature to match for the expected HTTP status or body text (if you want the 503 to be considered as UP)