Xcode Cloud setup failing without clear error

Good morning all,

We have an on-premise instance of Bitbucket (v8.19.25), using a custom port. We'd like to try using Xcode Cloud, compare to our current CI/CD provider.

Once I have to select my source control provider to grant access, I select Bitbucket Server. If I try to input the Host Name with, or without a port, I'm receiving this response:

{
    "message": "An error occurred",
    "details": [
        {
            "message": "An error occurred",
            "source": "host"
        }
    ],
    "actions": [],
    "recovery_actions": [
        {
            "dismiss_label": "OK"
        },
        {
            "feedback_label": "Provide Feedback",
            "feedback_component": "Xcode Cloud | New Bugs"
        }
    ],
    "should_show_error": true
}

Which, of course, doesn't help me in the slightest to understand why the connection fails. I do notice that the request to https://appstoreconnect.apple.com/ci/api/teams/<id>/setup-flow/scm-repositories in the web inspector already includes a host and port that I use to clone the repository locally. However, the web-based interface of Bitbucket doesn't include that port.

So, if I clone a repo: ssh://<bitbucket-instance>:<port>, but if I look at pull requests: https://<bitbucket-instance>`.

My colleagues have already determined that there shouldn't be any firewall rules or gateway limits, so Bitbucket should be reachable. No VPN either.

How can I continue to fix this?