Comment # 4 on bug 1124027 from Fabian Vogt

>  2 | download.opensuse.org-oss           | Update Repository (DEBUG)                   | Yes     | (r ) Yes  | Yes     |   99     | rpm-md | http://download.opensuse.org/debug/update/leap/15.0/oss 
>  9 | http-download.opensuse.org-c47d8002 | openSUSE:Leap:42.3                          | Yes     | (r ) Yes  | Yes     |   99     | yast2  | http://download.opensuse.org/distribution/leap/42.3/repo/oss/

You're using both 42.3 and 15.0 repos - that breaks your system.
Remove all 42.3 repos, run "zypper dup" and reboot.