Hi,
about a month ago Marcus Meissner reported to this list that
http://download.opensuse.org/update/leap/15.3/backports/ was not mirrored.
As of today, the mirroring is still not happening.
Example:
https://ftp5.gwdg.de/pub/opensuse/update/leap/15.3/backports/
https://mirrors.edge.kernel.org/opensuse/update/leap/15.3/backports/
are empty.