Cloudflare Pages Custom Domain error

Hi there,
I’m currently having an issue with Cloudflare Pages Custom Domains. I’ve got one project that’s working just fine but when I tried to set up a second the domain showed Inactive (Error) and is continuing to do so. I’ve since changed the DNS back to my server (I’m migrating the website in question to CFP), but before doing so I had a look at the API requests the dashboard is making, which uncovered the following error.

SSL Validation encountered the following errors: The requested hostname resolves to a Cloudflare zone that is not owned by you. HTTP domain control validation (DCV) cannot be completed. Update this hostname to resolve to your zone and PATCH to try again.

Obviously I do own the zone, and Cloudflare is even able to update DNS. In fact, the site that’s working is running on dashboard.mydomain.com and I’m trying to run this project on mydomain.com, but it refuses. Same with www.mydomain.com. Any ideas? Happy to provide CF staff with account/zone IDs etc as needed.