Error 503 using SSL gateway, trying to set up a redirect

Hi, I have the following domains hosted with OVH; I’m trying to set up a redirect from the first domain to the second and running into problems.

* hdsb-music-publishing.com
* hdsb.eu (the domain of my website, which is hosted at Elementor Cloud/google Cloud)

When I initially set up the redirection via the OVH control panel, it worked fine from http://hdsb-music-publishing.com/, but I got a connection error from https://hdsb-music-publishing.com/.

Assuming this was due to a lack of an SSL certificate on that domain, I set one up using OVH SSL Gateway. After configuring this, I appear to have the SSL certificate working correctly – State; IPv4 DNS state; and IPv6 DNS state all showing green on the control panel.

However, now I get an “Error 503: Backend unavailable” message.

Weird thing is, that error page is pulling the favicon from my website over at hdsb.eu, so there must be some kind of connection.

I’m at a loss on how to fix this, so any help and advice would be greatly appreciated.