The suite "stable-backports" does not accept source uploads. [was: cockpit_294.1-1~bpo12+1_amd64.changes REJECTED]

[Martin Pitt <mpitt@debian.org>]

Hello FTP masters,

Debian FTP Masters [2023-06-28  7:06 +0000]:
> The suite "stable-backports" does not accept source uploads.
>
> Mapping bookworm-backports to stable-backports.
> binary:cockpit is NEW.
> binary:cockpit-bridge is NEW.
[...]

I don't understand this. I first tried a source-only upload (as I usually do),
but that was rejected. Okay, that's because bookworm-backports is new and it's
the first cockpit backport. Then I tried the usual source+binary upload, which
resulted in the above rejection.

But surely we are not supposed to upload binary-*only* uploads? The source has
to be included, whereelse would it get them from?

Thank you for any enlightenment!

Martin