File ‘./repodata/c1693aa2553819d23a97ded730826b407097bd78264184c625fe70be636be944-deltainfo.xml.gz’ not found in the media ‘http://download.opensuse.org/update/leap/42.3/oss/’
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Maybe the mirror you use is down. By default your query is redirected to a mirror geographically closer to you. Wait a bit try again and if it continues then ask here again
Thanks TSU: not sure if zypper had picked up the change but the non-oss URL had reverted to plain /non-oss/ in my list of repositories - the date of the change to the oss directory is today but I have not run zypper since the change.