If you look at the contents of /etc/ostree/remotes.d/fedora.conf you should see the following:
$ cat /etc/ostree/remotes.d/fedora.conf
[remote "fedora"]
url=https://ostree.fedoraproject.org
gpg-verify=true
gpgkeypath=/etc/pki/rpm-gpg/
contenturl=mirrorlist=https://ostree.fedoraproject.org/mirrorlist
You can see the contenturl is using the mirrorlist= format.
The mirrorlist format that is supported by ostree is just a “plain text file of newline-separated URLs” (ostree.repo-config). And I believe it has always pointed to a single CloudFront domain. (See the history on the file in the Fedora package - Tree - rpms/fedora-repos - src.fedoraproject.org)
The CloudFront domain is a CDN for the ostree content, which is why you see Index of /ostree/repo because the ostree repo content lives there.
Now this doesn’t explain the error you are seeing, but hopefully provides a little more context.
I would ask if the error is repeatable every time Discover is used. Or if it is repeatable when doing rpm-ostree upgrade from the command line? Can you resolve https://ostree.fedoraproject.org/mirrorlist when using curl from the command line? Are there additional logs from journalctl -b that might describe why the name resolution is failing?
In short, some additional information is needed to understand why the error is occurring and what triggers the error.